Middle Eocene Ostracoda from Baja California Sur, Mexico

One genus and six new species of ostracodes are described from the Bateque Formation on the Pacific Coast of Baja California Sur, Mexico. Planktonic foraminifers indicate a mid Eocene age and the whole assemblage is characteristic of a shallow warm-water environment. Paijenborchella mezquitalensis sp. nov. is the second record of the genus Paijenborchella from the Eocene of North America. Except for this species and the new genus Bajacythere, the ostracode association has strong affinities with those described from the lower Tertiary Gulf Coast region.


INTRODUCTION
During the last few decades, geological research in the Baja California Peninsula has been focused on the study of deep sea Miocene and Pliocene sequences that were deposited during or immediately after the formation of the Gulf of California. The objective of those efforts has been to understand the origin and evolution of the gulf in relation to neotectonics.
A g r e a t p a r t of t h e Peninsula of Baja California, particularly its western slope, is covered by lower Cenozoic outcrops that have been assigned to different formational units: the Tepetate Formation (Upper Cretaceous to mid Eocene), the Malarrimo Formation (Paleocene), the Ballenas Formation (Paleocene), the Santo Doming0 Formation (Paleocene and only recognised in boreholes), the Sepultura Formation (lower Eocene), a n d the Bateque Formation (lower and mid Eocene).The sediments referred to as the Bateque (Mina-Uhink,l957) a n d / or the Tepetate (Heim, 1922) Formations include m a r i n e fossiliferous b e d s containing abundant microfossils.
The main objective of this paper is to document the Ostracoda from t h e Bateque Formation. The paleozoogeographic and biostratigraphic significance of this fauna is included.

MATERIAL AND METHODS
T w o localities of t h e Bateque Formation yielded t h e ostracodes discussed in this paper. The first locality (IGM-2,515) is located 32 k m southwest of San Ignacio, Baja California Sur, Mexico (Fig. l), and is the type locality of the Bateque Formation (Mina-Uhink, 1957). Sampling was made in a section at the northern side of Arroyo San Ramon, near Rancho Batequi d e l Monreal (Rancho Bateque), approximately 2.5 k m northeast of latitude 27'00" a n d longitude 113"OO'W (San Jose d e Gracia Quadrangle, Baja California Sur, Mexico, 1: 50,000 chart number G12A64, INEGI, 1983). The measured section is 52 m thick a n d consists mainly of m e d i u m to very fine micaceous, feldspathic s a n d s t o n e w i t h i n t e r b e d d e d fossiliferous lenses and siltstones. The base of the formation is not exposed; the unit is unconformably overlain by the Miocene volcanics of the San Isidro Formation. The ostracode collection comes from 16 to 40 m above the base of t h e outcrop, w h e r e fossiliferous lenses b e a r i n g discocyclinids occur.
The second locality (USGS-84TC51) is a north-facing 35 m high bluff at the southern side of Arroyo Mezquital (Fig. I), just south of the dirt road leading to San Juanico (13.5 km to north 26'14" -112"23'W, San Isidro Quadrangle, Baja California Sur Mexico, 1: 250, 000 chart number NG12-4, CGSNEGI, 1982). The ostracode samples were collected at the base of a 30 m thick section, that consists of very weathered reddish-gray mudstone and very fine-grained sandstone beds. From the two localities, 30 samples were examined for ostracodes. Detailed geological m a p and information including both localities was published by coworkers (1985,1987). Mina-Uhink (1957) studied several localities from this area and considered that the Bateque Formation was deposited during the early and mid Eocene, based on the presence of Turritella pachecoensis Stanton, Morozovella aragoneiisis (Nuttall) and Acarinina crassata (Cushman). Carrillo d e Isolbi (1976) assigned a later early Eocene and mid Eocene age to the outcrops exposed in the PEMEX San Ignacio-Cadeje Project, based on the planktonic foraminifers and calcareous nannoplankton.

BIOSTRATIGRAPHIC FRAMEWORK
From the Arroyo San Raymundo section, located near our Arroyo Mezquital locality, McLean & Barron (1988) recovered a diatom assemblage assigned to the Triceratium iriconspicuuni var. triloba Partial Range Zone, equivalent to a later mid Eocene age.
The presence at locality USGS-84TC51 of Morozovella aragoiiensis a n d Tririicorotaloides topileiisis (Cushman), as so c i a te d with T u r b o r o t a 1 ia ce Y r o a z u 1 e n s is p o m e r o 1 i (Toumarkine & Bolli), "Globigcrinoides " higginsi Bolli and Globigerinatheka subconglobata subconglobata (Shutskaya), indicates an age earlier than mid Eocene, equivalent to the upper Hantkenina nuttalli -Globigerinateka subconglobata subconglobata zones of Toumarkine & Luterbacher (1985).

SYSTEMATIC PALAEONTOLOGY
Only new taxa are systematically treated here. The reduced n u m b e r of specimens, poor preservation a n d / o r the presence of closed carapaces, did not allow the assignment of H a z e 1 in a, Po k o r n ye1 1 a, B u n t o n ia, " Trig i n g 1 y m u s ", Bairdoppilata spp. Remarks. The material is poorly preserved, and mostly consists of carapaces. Two groups, based on shape can be recognised; one group is of large, subpyriform and inflated forms, similar to Bairdoppilatn taxodoiitn Howe & Law (1935).
Forms in the other group are also large sized, but elongated, sharply pointed posteriorly and broadly pointed anteriorly.
They resemble Bairdoppilatn zwiioiii Howe (1951 Diagnosis. Carapace small, thick, ventrally inflated with a deep median sulcus and longitudinal ridges; wide anterior fringe bearing at the RV six larger pits parallel to the anterior margin; at the LV a n anterior flange with a n irregularly dentate fringe; dorsal margin obscured by four strong like-spine projections. Description. Carapace small, thick, highest at the anterior cardinal angle; widest posterior to the center. Dorsal margin of the LV is almost straight and slightly inclined, dipping posteriorly with four strong spine-like projections, the dorsal margin of the RV straight; ventral margin strongly convex; anterior margin broadly rounded, bearing at the RV six large parallel pits and at the LV, the anterior flange being a n irregular d e n t a t e fringe. Posterior margin subtriangular, with caudal process. Surface ornamentated by a deep median sulcus, longitudinal ridges, pits, and irregular reticulate pattern; sulcus extends ventrally from anterior cardinal angle, terminating against a prominent ventral longitudinal ridge; between this ridge and ventral margin, a short, weak ridge; a shorter prominent ridge bridges the sulcus near center line; surface anteriorly smooth, most part of the valve coarsely and evenly pitted; mid-posterior surface rough, following a n irregular reticulated pattern. Cardinal tubercle prominent. Inner lamella wide, line of concrescence and inner margin coincide throughout; hinge of the LV with an anterior large socket and postjacent crenulate tooth connected by narrow crenulate median element to subrounded posterior socket. Radial pore canals straight and numerous; muscle scars obscured. Sexual dimorphism strong, males larger than females.  (Stephenson, 1944) (PI 1 figs 7-8) 194.1 Cytlicwis zuaslibirriii Stephenson: p. 452,pl. 76,fig. 8. Diagnosis. Carapace m e d i u m sized, thick, robust, subrectangular in lateral view. Dorsal and ventral margins nearly straight and gently converging towards subangular posterior end; anterior margin broadly rounded. Anterior and posterior margins bear denticulations that are most prominent ventrally. Dorsal margin slightly obscured by a row of large, rounded, nearly pointed nodes. Surface coarsely reticulated, deep and polygonal pits surrounded by ridges of nearly equal height bearing blunt rounded nodes at their intersections. Anterior with five to six large blunt rounded nodes. Eye tubercle very prominent, nearly surrounded by blunt nodes. Hinge of RV with a blunt, anterior tooth connected by a narrow, smooth, straight dorsal furrow to a large subpyramidal posterior tooth, LV complementary. Radial pore canals, numerous and straight. Muscle scars obscured.

Mi.
Material. 33 specimens. Remarks. In spite of the fact that the Bateque Formation specimens have lost most of their surface spines, due to preservation effect, they appear to be identical to A. waskhmriri (Stephenson, 1944) anterodorsally with a thick s u b r o u n d e d projection extending over the RV; denticulate anterior and posterior margins; smooth a n d polished w i t h few a n d widely distributed, apparently open normal pore canals; two submedian parallel ridges; narrow ventral ridge extending from anteroventral margin to posteroventral end; moderate broad anterior rim; eye tubercle present; moderately broad inner lamella, narrowing ventrally; moderate number of long, wavy marginal pore canals; holamphidont hinge; four elongated adductor scars in a vertical row; V-shaped frontal scar. Remarks. This g e n u s is d i s t i n g u i s h e d from other trachyleberidinid tricostate genera by its broad inner margin. It resembles Occdtocythereis but differs by its larger size, the presence of a prominent submedian ridge and simple, long a n d s i n u o u s marginal p o r e canals. Phacorhabdotus is similar, but its carapace is much shorter, its anterior frill is thin, it has no marginal ridge, its muscle swelling is pronounced, and its pore canals are paired. In its general outline and the characteristic strong hinge, Bajacythere resembles Recent Australian Cythereis (Cythereisi cristatella Brady (1866), later reported by Brady (1880) as Cythere cristatella from Recent sediments off Booby Island, southern New Guinea.
(P1 1 figs 9-13) Derivation of name. After Baja California Sur, Mexico. Diagnosis. A tricostate species characterized by a smooth and polished surface with small deep pits, moderately broad inner margin; denticulate anterior and posterior margins, valves asymmetrical. Holotype. LV female IGM-328-Mi Material. 40 specimens. Locality and horizon. Locality USGS-84TC51 southern side of Arroyo Mezquital, near San Juanico (26'14" -112"23'W), Baja California Sur, Mexico. Bateque Formation, mid Eocene age. Unconsolidated foraminifera-rich, fine-grained sandstone. Description. Carapace with greatest height at the anterior cardinal angle; greatest width at the posterior end of the submedian ridge; LV with a thick subrounded hinge earlike projection anterior to the eye node. Narrowly cuneiform in dorsal view, compressed in the anterior and posterior with truncated extremities. Dorsal and ventral margins almost straight in lateral view; anterior margin broadly rounded with 20-25 denticles; posteroventrally denticulate, postdorsal and postventral margins forming a blunt angle. Surface smooth and polished; two longitudinal and parallel ridges extending about half the length of the carapace, rising from anteromedial area and terminating posteromedially; marginal rim low; ventral ridge oblique, extending from the mid-anteroventral margin to the posterior end of the median ridges; normal pore canals open and moderate in number. Inner lamella moderately broad, narrowing ventromedially; line of concrescence and inner margin coincide throughout; marginal pore canals simple, long, w a v y w i t h distal thickening; selvage prominent, peripheral. Hinge in the RV with a strong tooth having a higher large and rounded distal part, a lower reniform proximal part a n d a large postjacent socket anteroventrally opened to the inside; median groove straight and smooth; posterior tooth large rounded. LV complementary. Sexes distinct, males longer and lower than females.

Genus Hazelina MoosJ966
Hazelina sp. (P1 1 figs 14-16) Description. Small sized carapace, subrectangular in lateral view, highest at the anterior cardinal angle, widest at 1 / 3 of the posterior margin. Dorsal margin nearly straight, ventral slightly convex at center, both converging toward the posterior. Anterior end, broadly and obliquely rounded and fringed, w i t h a parallel to it rim carrying s m a l l anteroventral denticlcs. Posterior end compressed, dorsally Subangular, slightlv convex ventrally. Valves surface covered with a polygonal reticulated pattern. Three low ridges almost para1 lel extend obliquely backward from about 1 / 3 behind the anterior margin to a point near the posterior cardinal angle, where they join by means of a short, and weak ridgc forming a strong angular projection. Eye tubercle small and indistinct. Internal characters not observed. Hypotypes. C ICM-334-Mi; C ICM-335-Mi and C ICM-336-Mi. Material. 11 specimens. Remarks. This specirv, is similar to Ilnzdiiia cotdeycrecket~sis (Gooch, 1939), but differs in that the submedian ridge in Description. Carapace small, thinly calcified; subrectangular, laterally very compressed; highest at the anterior cardinal angle, widest at the posterior end of the dorsal ridge; valves slightly asymmetrical, LV moderately overlapping RV anterodorsally. Dorsal and ventral margins nearly str<iight converging toward posterior end. Anterior margin rounded and denticulated throughout with a thick, low rim; posterior margin narrower, angular below the middle bcaring five denticles. Surface smooth, normal pore canals widely spaced; a short, low ridge extends from a depression located behind the eye tubercle a n d r u n s subparallel to the dorsal margin reaching the posterior cardinal angle where it turns down sharply forming a n angular projection. A short ventral ridge extends 1 /3 from the anterior end, running obliquely upward to end abruptly 1 / 3 beforc reaching the posterior margin forming a blunt projection. Internally, the valves are shallow; inner lamella moderately broad, narrowing ventromedially and incurved; line of concrescence a n d inner m a r g i n coinciding throughout; marginal pore canals straight and moderate in n u m b e r . Hinge of the RV consists of a large, sharply anterior tooth behind which is a rounded deep socket; median groove straight and finely crenulated; Remarks. In its general outline, this species resembles Occcrltocyfliereis broussardi (Howe & Chambers, 1935), but differs in that the species of Baja California is smaller, has a more depressed carapace, and the posterior end of both valves is 'ingular below the middle, and has fine denticles; it has also imall, rounded denticles throughout the greater part of thc anterior end; dorsal rim is not present. Distribution. Locality USGS-84TC51.
Subfamily Hemicytherinae Puri, 1953Tribe Thaerocytherini Hazel, 1967 Genus Hevmanites Puri, 1955 Herriranites batequensis Carrefio & Cronin sp. nov.  Anterior margin broadly and obliquely rounded, fringed with numerous small denticles, posterior margin truncated in the LV, at the RV its dorsal portion is concave, in both valves the ventral portion bears five marginal spines and two in the dorsal portion. Subcentral swelling low, surface of valves foveolate with strong coarse pits projecting over the dorsal margin, with a very low dorsal ridge, a rounded knob at the posterior cardinal angle. A low, rounded submedian ridge starting at the anterior margin runs back to the central swelling where it becomes more evident, turning upwards and ending at the rounded knob of the posterior cardinal angle. Another ridge runs parallel to the ventral margin in the RV, whereas in the LV it runs obliquely to the ventral margin. Inner lamella moderately wide, line of concrescence and inner margin coincide, radial pore canals straight and numerous. Central muscle scars consist of a posterior vertical row of four elongated scars, with two closely situated frontal scars. Hinge of RV consists of a high conical anterior tooth, a postjacent socket connected by a straight groove to a lobed posterior tooth, hinge of LV complementary. Marginal pore canals numerous, simple and straight, dorsally widely spaced, anteriorly paired, and ventrally numerous and closely spaced.

Dimensions (mm)
L H Remarks. This species was placed in the genus Hermanites because of the general outline, ornamentation and hinge; nevertheless, the dorsal ridge is not well-developed. Also, an unusual low submedian ridge is present, possibly due to the replacement of foveolate with reticulated pattern of ornamentation. Hermanites batequensis sp. nov. resembles H. paijenborchiana (Keij, 1957)  Description. Carapace subquadrate in lateral view with an almost straight dorsal margin and slightly sinuous ventral margin; greatest height at the anterior cardinal angle; greatest width at the posterior end of the projecting ribs. Anterior margin almost evenly rounded, wider dorsally than ventrally, with about 20 denticles along a heavy marginal ridge. This ridge merges with the anteroventral margin, ending dorsally in a moderate eye tubercle; posterior end short, laterally compressed, angled subventrally, concave above, subrounded and with five strong spines. In dorsal view, sides are parallel and inflated; LV overlapping right at the anterodorsal angle. Surface covered with weak reticulation, the ventral ridge rising at the anterior ridge and runs almost parallel to the ventral In internal view, valves are deep, hinge consists of a large rounded socket with a postjacent small subconical tooth, the median element is a smooth straight bar that ends in a very deep, large rounded socket. The adductor muscle scars consist of a vertical row of four scars and two frontal scars obscured by a shallow depression at the muscle platform. Sexual dimorphism, females higher than males. Remarks. The general outline of this species resembles the female carapace of Keij's (1957) Bradleya kaassckieteri from the upper Eocene of Belgium. It differs from the latter in the presence of two submedian ridges and in the pattern of the smaller ones. Distribution. Locality USGS-84TC51.

Jugosocythereis monrealensis
Carreiio & Cronin, sp. nov. (P12 figs 9-12) Derivation of name. After Rancho Batequi del Monreal. Diagnosis. Carapace inflated, covered with numerous, large and irregular reticulated pattern; the median ridges characteristic of the genus very weakly developed. Holotype. LV female IGM-348-Mi Material. 27 specimens. Locality and horizon. Locality IGM-2,515, 32 km southwest of San Ignacio, Baja California Sur, Mexico, northern side of Arroyo San Ramon, near Rancho Batequi del Monreal. Approximately 2.5 km northeast of the intersection of 27'00' latitude N -113"OO' longitude W. Mid Eocene age. Medium to very fine-grained, micaceous sandstone. Description. The carapace is heavily calcified, subrectangular in lateral view; dorsal view irregularly sagittated with the anterior and posterior ends depressed. Laterally, dorsal margin nearly straight; ventral margin slightly convex; greatest convexity just anterior to the middle; greatest height at the anterior cardinal angle; greatest width at the mid-center because of the projection of the subcentral swelling. Anterior margin flattened and somewhat obliquely rounded, bearing a large rim likefringe projection, more evident from the mid-anterior to the cardinal angle, with approximately 23 small denticles, more conspicuous at the anteroventral margin. Posterior dorsal margin truncated and slightly concave, resulting in a blunt angle with the posteroventral margin, which has four blunt, large spines. The entire valve surface is covered by numerous, large and irregular fossae formed by thin murae, forming a reticulated pattern. Six large fossae occupy the anterior margin. Below the eye tubercle and behind the most anterodorsal fossae, there is a sulcus which is bordered by the emerging oblique dorsal ridge. This ridge extends back, obscuring the posterior part of the dorsal margin and ending in a strong lateral projection in front of the posterior cardinal angle. At this point, it turns downward running parallel to the posterodorsal margin almost to join the terminus of the ventral ridge. The ventral ridge is subparallel to the ventral margin. The characteristic median ridge of the genus is weakly developed in some specimens (females?), where it forms a short reticulated row of tubercles in RV. In other specimens (males?), one upper submedian ridge, emerging from the flattened anterior margin, runs slightly upward and disappears in front of the posterior cardinal angle. A second low submedian ridge, more evident in the RV and in juveniles, runs subparallel to the upper one, disappearing at about 1 / 6 of the length from the posterior end. In the LV, this ridge is weaker and ends near the muscle platform, where a concentric reticulated pattern occurs.
Internally, the valve is deep. In the RV, the hinge has a strong conical anterior tooth and a small subovate postjacent socket with a medial smooth groove ending in a large ovate posterior socket. Adductor muscle scars pattern partially obscured, consists of an oblique row of four scars; there are numerous straight radial pore canals.

Dimensions (mm)
L H Remarks. This species is similar to several other species described from the Gulf Coastal Plain and Florida. In dorsal view, it is similar to Jzigosocytlzereis zddnrrgeizsis (Howe & Law, 1936) from the mid Oligocene, Vicksburg Group.
Byram, Hinds County, Mississippi and J . bialntn (Howe, 1951) from the mid Eocene, Avon Park limestone at Levy County, Florida. Nevertheless, this species is distinguishable by its conspicuous, flattened anterior margin and somewhat obliquely rounded margin, which bears a large rim like-fringe projection, carrying six large fossae. In addition, the characteristic median ridges of the genus are weakly developed in the species from the Bateque Formation. Distribution. Locality IGM 2,251 Allotype RV male IGM-350-Mi 0.922 0.544 Tribe Pokornyellini Puri, 1974Genus PokomyelIa Oertli, 1956 Pokornyella sp.
(P13 figs 1-3) Diagnosis. Carapace medium size, heavy, thickest along median portion and near middle, subrectangular in lateral view, lenticular in dorsal view. Dorsal margin straight in LV, slightly curved in RV, ventral margin slightly sinuous, anterior margin broadly and obliquely rounded, posterior e n d blunt bearing t w o small ribs above where t h e posterodorsal slope is concave. A lower small rib extends from the posterior end, nearly parallel to ventral margin; near the anterior end, it forms a distinct ventral ridge. Surface ornamented with deep oval to quadrate coarse reticulation. Eye tubercle small. Internal characters were not observed because of the absence of isolated valves. Hypotypes. C IGM-353-Mi; C IGM-354-Mi; C IGM-355-Mi. Material. 47 specimens. Remarks. Disarticulated valves were not f o u n d a n d detailed observation of the internal characters ~7 a s not possible. The abundance of well preserved carapaces permits comparison with other mid Eocene Pokovrryella, particularly with similar species described from the Avon Park limestone, Levy County, Florida as P. cribaria (Howe, 1951) and P. bellzrla (Howe, 1951). The Bateque Formation species is bigger, more strongly ornamented, much slender in dorsal view and less quadrate in lateral view than the other species. In our opinion, this species represent a new taxa, but we prefer to leave it in open nomenclature because at the moment we can not fully describe it. Distribution. Locality IGM 2, 515 Subfamily Buntoniinae Apostelescu, 1961 Genus Buntonia Howe, 1935 Buntonia sp. (P1 3 fig 4-6) Remarks. Due to t h e scarce material a n d its poor preservation, it was impossible to identify this form to a specific level. Nevertheless, this species appears to be related to Burrtoilia shubutaensis Howe in Howe & Chambers (1935), except for the fact that Runfonia shiibzitaeiisis-is larger. Hypotypes. C IGM-356-Mi; C IGM-357-Mi; C IGM-358-Mi. Material. 19 specimens. Distribution. Locality IGM 2, 515 Subfamily Campylocytherinae Puri, 1960Tribe Leguminocythereidini Howe, 1961 Genus Triginglymus Blake, 1950 "Triginglyrnus " sp.
(PI 3 figs 7,8) Diagnosis. Carapace elongated in lateral view, lanceolated in dorsal view with compressed posterior end; dorsal and ventral margins almost straight. Anterior end broadly and evenly rounded carrying several denticles in the midventral portion; posterior margin subangular with the dorsal portion slightly concave, ventral portion with six small rounded spines. Valves covered anteriorly with two low parallel rims, which continue along ventral and dorsal margins; surface coarsely reticulated with several prominent sinuate longitudinal ridges. Eye tubercle well developed. Inner characters were not observed because material consists of carapaces; nevertheless, it was possible to observe the LV hinge in a broken specimen, which consists of a large, rounded anterior socket and a postjacent b l u n t tooth followed by a triangular anti-slip tooth projection connected to an ovate socket by a median smooth bar. Males are more elongated than females.
Remarks. The hinge structure observed in this species suggests inclusion in t h e g e n u s Triginglymus.
Ornamentation is similar to that exhibited in T. longicostata (Blake, 1950) described from the mid Eocene of the Gosport sand from Clarke County, Alabama; nevertheless, the species from the Bateque Formation is bigger, more coarsely reticulated and with a denticulated anterior mid-ventral margin. We left this species in open nomenclature due to the lack of valves. Hypotypes. C IGM-359-Mi; LV IGM-360-Mi. Material. 7 specimens.
(P13 figs 9-11) Diagnosis. Carapace small, inflated and ovate in dorsal view, dorsal margin straight, ventral margin concave. Anterior margin obliquely rounded, posterior margin in RV truncated dorsally, broadly rounded from mid-dorsally to ventrally; LV is dorsally concave a n d slightly acute; ventrally LV is obliquely rounded. Surface entirely covered with a quadrate to rectangular reticulated pattern. Hypotypes. C IGM-361 -Mi; LV IGM-362-Mi; RV IGM-363-Mi. Material. 9 specimens. Remarks. This species is very close to Loxoconclza marionensis Puri, 1957 from the upper Eocene of the Crystal River Formation of Florida in its general outline, except for the presence in the Bateque material of a low dorsal keel. L. cocoaensis Krutak, 1961 is also similar but, in that species, the dorsal ridge is blade-like, instead of rounded as in Loxoconclia sp. from Baja California. Distribution. Locality USGS-84TC51.
Family Paracytherideidae Puri, 1957Genus Paracytheridea Muller, 1894 Paracytheridea sp. (P13 fig 12) Diagnosis. Carapace subrectangular in lateral view, strongly alated. Dorsal a n d ventral margins straight, anterior margin broadly rounded, posterior end draw out into a distinct caudal process. Each valve bears a strong ala which projects from the midventral zone, terminating at about 1 / 3 the shell length from the posterior. Surface ornamented with a pattern of low, anastomosing ridges; anteriorly a group of four ridges radiates out from a point near the antero-ventral zone. Internal features typical for the genus.
Remarks. Compared to other Eocene Paracytheridea species, the material from the Bateque Formation is similar to P. basfropensis Stephenson, 1947, but the former has a more elevated ventral alar projection, and a subacute posterior end. It is very closely related to P. belhavenensis Howe & Chambers, 1935, b u t detailed comparison to the type specimen of this species shows that the distribution of the valves surface ridges is different a n d the subcircular depression located in the anteriomedian zone is not evident in the Bateque Formation species. Distribution. Locality IGM 2, 251.
Family Paracyprididae Sars, 1923 Genus Paracypris Sars, 1866 Paracypris sp. Diagnosis. Medium-sized carapace, smooth, elongated, subpyriform in lateral view, dorsal margin arched forming an angular peak at mid-portion, more conspicuous in LV, ventral margin straight to slightly concave particularly at the center, posterior end is narrow, subacute. LV overlaps RV throughoc:, more strongly dorsally and ventrally. Hypotypes. C IGM-366-Mi; C IGM-367-Mi. Material. 21 specimens. Remarks. The material consisted only of carapaces. This species resembles Paracypris licina (Huff, 1970), but differs in that it is smaller, a n d t h e dorsal margin has a small concavity at the anterior cardinal angle. P. franquesi Howe & Chambers (1935) is much larger, and the dorsal margin has an angular peak at the anterior cardinal angle instead of at the mid-portion. Distribution. Locality IGM 2, 251.

Family Pontocyprididae Muller, 1894
Genus Argilloecia Sars, 1866 Argilloecia sp. Diagnosis. Carapace elongated, highest just anterior to the middle. Anterior margin irregularly rounded, almost angulated just above the middle. Dorsal margin straight or very gently convex in t h e middle, curving gently downward at each extremity. Ventral margin very slightly convex along its whole length. Posterior margin obliquely rounded, blunt. Muscle scars typical of the genus. Hypotypes. C IGM-368-Mi; C IGM-369-Mi; C IGM-370-Mi. Material. 12 specimens. Remarks. This species resembles Argilloecia in all respects except that it has reversed hinge overlap. The material of the Bateque Formation only consists of carapaces so the muscle scars could only be observed t h r o u g h t h e semitransparent carapace. The marginal area w a s not observed in detail. Distribution. Locality USGS-84TC51

D I S C U S S I O N
The foraminifers associated with the ostracod fauna of the Bateque Formation are characteristic of the Haiitkeiiiira iiuttallz Interval Zone and Globigerinatheka subcoiiglobatn suhconglohata Concurrent Range Zone, suggesting a mid Eocene age of 50.3 -54.1 Ma (Toumarkine & Luterbacher, 1985). Sorensen (1982) a n d McLean a n d coworkers (1987) reported Bateque sandstones, yielding a b u n d a n t fragmented shallow-water fossils, in a predominantly deepwater assemblage interpreted as significant downslope transport. The diatomaceous deposits suggest outer-shelf to upper-slope paleodepths affected by strong upwelling (McLean & Barron, 1988).
At locality IGM 2,215, the presence of Pseudophragmina (Proporocyclina) f l i n t e n s i s a n d Amphistegina lopeztrigoi indicates a shallow, warm-water environment, deposited at a depth of no more than 100 m (Vaughan, 1945); whereas Thaerocytherinii, that constitutes more than 90%, reinforces the interpretation that deposition occured in an outer-neritic environment. Dominance of carapaces vs. valves could indicate rapid burial in a deeper part of the basin.
At USGS 84TC51 locality the exposed lithology consists mostly of very f i n e s a n d s t o n e w i t h interbedding fossiliferous lenses, locally rich in well preserved stromatolites, coralline algae, foraminifers, calcareous sponges, gastropods, and bivalves (Squires & Demetrion, 1989, 1990a. No deep-water ostracods were found and t h e assemblage is d o m i n a t e d by thick-shelled Trachyleberdidae (Bajacythere, Occultocythereis, Jugosocythereis), suggesting an outer-neritic environment.
The associated benthic foraminifers are characteristically outer-neritic morphotypes.
The ostracod assemblage is a mixture of faunas from different paleobiogeographic provinces. In general, the whole ostracod assemblage has strong taxonomic affinities with the Eocene of the Gulf Coast and the Mediterranean. This suggests a marked influence of the Tethyan corridor as a means for tropical species latitudinal dispersal during the Eocene.
The species Paijeiiborcliella t r i g o m has previously been reported from the Californian Province and, together with the six new species (including Paijrtiborchelh mqilitnleiisis) and the new genus Bajacythere, constitutes an isolated record of t h e Eocene ostracod fauna in the area. Paijeiiborcheila is a typical Mesozoic and Paleogene genus f r o m Europe a n d its presence in America's Eocene s e d i m e n t s strongly reinforces its Tethyan origin. Paijelzhorchella is also present in California during the Late Cretaceous (Holden, 1964).
The n e w g e n u s Bajacythere raises a n interesting paleogeographic problem. This species is similar to species found in the Equatorial Pacific Ocean at Enewetak Atoll in u p p e r Miocene to Holocene s e d i m e n t s (Cronin, unpublished data) and also seems to be closely related to Maastrichtian species reported from Europe (i. e. Cythereis aiiorchidea Veen, 1935). The Tethyan influence on the paleogeographic distribution of this genus is presently not certain.
In s u m m a r y , the Eocene Baja ostracod assemblage provides important new data to our understanding of circum tropical Tethyan ostracod paleobiogeography.

A C K N O W L E D G M E N T S
A. L. Carreiio gratefully acknowledges Dr. J. Pojeta, for the provided facilities during my stay at the United States Geological Survey, Reston, Virginia, supported by Consejo Nacional d e Ciencia y Tecnologia. Direccion General d e Asuntos del Personal Acadhmico, UNAM and Universidad Autonoma d e Baja California Sur, gave logistic support.
Technical assistance of M. Alcayde-Orraca, from the staff of the Instituto d e Geologia, w a s also very valuable. I a m indebted to Margarita Cronin for all her kindnesses. We thank Dr. J. E. Hazel (Louisiana State University), Dr. K. L. Finger (Chevron Oil Field Research C o m p a n y ) a n d R.
Blodgett (United State Geological Survey) for their patience in improving early drafts. Thanks also to Alvin M. Phillips, Curator of the Henry V. H o w e Collection (Museum of Geosciences Louisiana State University) for his help. The authors are also indebted with two anonymous reviewers.

Manuscript received April 1992
Manuscript accepted August 1993