The structure and taxonomic position of Millettia Schubert, 1911 (Foraminiferida)

The study of opened specimens of Millettia species confirms the presence of septula subdividing the individual chambers. Toothplates are present between the individual septular foramina, and the chamber foramen. Polished and etched sections show the test to be bilamellar, but secondary lamination is absent. The toothplates are monolamellar and made up of inner lining, each one continuous with the septula which are formed by doubled-up inner lining. Laterally, the internal chamber walls are covered by a web-like coating of supplementary inner lining, continuous with the septula. X-ray diffraction shows the genus to be calcific. The genus is revised to include M. tessellata, M. limbata and the newly described M. ipsithillae and M. polyxenae. The monotypic Millettiidae are considered incertae sedis within the Rotaliina.

described Sagrina (?) tessellata, the seemingly innocuous question mark would prove to be the steady companion of this species for a very long time indeed. The peculiar test morphology and taxonomic confusion concerning the naming of the taxon has continued to be a problem for systematists.
Howchin (1889) reported specimens of S. lirnbatu from -Tortonian deposits (Miocene) of Muddy Creek, Victoria, Australia Millett (1903) discussed Sagrina tessellatu alongside S. lirnbata Brady, 1884 and noted that the chambers are divided by transverse septa. He pointed out that this morphological peculiarity was incompatible with the genus Sagrina and that a separate genus might have to be created for them.
In 1911, Schubert proposed the genus Millettia to accommodate Brady's S. tessellata, having encountered a single specimen in material from the Pliocene ofthe Bismarck Archipelago. Silvestri (1912) pointed out that thename Millettiu was preoccupied by Duncan, 1889 and Wright, 1899 and proposed Schubertia as a replacement name. Cushman (1929) presented a detailed discussion of the history and actual morphology of the by now called Schubertia tessellata. He demonstrated the presence of a pronounced dimorphism in Schubertia. The juvenile chambers in the megalospheric generation are usually biserial and already septate, while the microspheric juvenile chambers are rectilinear and simple. In Cushman's opinion, Schubertiu was derived from Siphogenerina or Rectoboliuina.
Galloway (1933) classified Schubertia in the Uvigerininae. Hofker (1951) restricted the genus Sagrina to include S. tessellata only. His thorough study showed for the first time the presence of transverse septa in the rectilinear chambers and of toothplates between the test by describing them as infoldings of the wall at the level of the septa, both horizontallty and vertically (Fig.1).
Loeblich & Tappan (1955) selected a lectotype for Schubertia tesselluta, and corrected Cushman's description. They described the early stage of S. tessellatu as being biserial and all the chambers up to the second rectilinear one as non-septate. Reiss (1963) created the subfamily Schubertiinae to accommodate the genus.
In 1964, Loeblich & Tappan presented evidence to support the reinstatement of the nameMillettia. The senior homonyms all turned out to be either non-available names or spelled slightly differently, i.e. Milletia instead of Millettia. Since Schubertia was preoccupied as well, Millettia was adopted as the correct name. In the description of the genus, the hexagonal pattern which marks the outer walls of the test is said to be caused by the junction of the chamberlet walls with the outer wall, the chambers being divided by horizontal and vertical septa. He tried to explain the tessellated nature of the Fig 1. A reproduction of the schematic drawings presented by Hofker (1951 Description. Test free, elongate, cylindrical, coiling axis commonly arcuate, curved nature of the test due to skew joining of the chambers, circular in endview, periphery regularly constricted at the junction of thechambers, first two chambers biserial, later chambers in a rectilinear series; chambers cylindrical, chambers straight, width gradually but only slightly increasing, rapidly increasing in height; sutures circular, not very clear, slightly depressed; aperture terminal, stellate, on a low neck with a somewhat thickened,everted lip, lip marked by grooves; chamber lumen subdivided by septula of doubled inner lining, lateral wall covered by a web-like thin tessellated pattern, made up of inner lining, number of septula per chamber increasing during ontogeny, septula thin, each pierced by a central foramen, successive septular foramina connected by a toothplate originating under the septular foramen, and fusing with the simple septular foramen, at the septa1 end fusing onto the foramina1 everted lip; toothplates almost triangular, slightly curved into a trough, at the bottom rather narrow, commonly both edges free but one edge may fuse with the lateral chamberwall later in ontogeny and form a single vertical partition in the lumen; wall calcitic, hyaline, no secondary lamination, ornamented by a superficial tessellated pattern, only the tesselae perforate. Remarks. A clear dimorphism is present in the species. the microspheric generation differs from the described megalospheric one in the smaller initial part of the test, in possessing an aperture which is present ona much lower neck, and which is obstructed by guttae originating in the thickened lip overhanging the aperture. Furthermore, the test bears a single, conspicuous longitudinal constriction, running continuously over all chambers. This constriction may be reflected in the lumina of the chambers by the presence of a protruding plate, which in the later chambers fuses laterally with the toothplates. Although it is continuous with the inner lining and eventually also with the toothplates, it should be distinguished from the latter.

Revets
Type locality. Macassar Strait, 45fms, ex Earland coll. Description. Test free, cylindrical, circular in endview, axis of the test arcuate, except for the first few chambers arranged in rectilinear series; chambers cylindrical, with an almost constant diameter, becoming longer throughout ontogeny; sutures circular, depressed, not well defined; aperture terminal, on a slender straight short neck without an everted lip, but clearly grooved; wall calcitic, hyaline, porosity unclear, ornamented partially by an ill-defined hexagonal low network of raised ridges, close to the apex of each chamber becoming scale-like in the form of stacked tiles. Remarks. Differs from the other species by being very elongated and slender, by possessing proportionally longer chambers, a very fine covering of the test wall, and by possessing scale-like protrusions close to the apex of each individual chamber. (Brady, 1884) ( Description. Test free, elongate, laterally compressed, cylindrical, elliptical in endview, a single fold running along the entire test gives it a nicked appearance, periphery flush, axis of the test may occasionally be arcuate, normally straight, earliest part may be biserial but most chambers arranged in a rectilinear series; chambers subcylindrical, straight, slightly if at all inflated, gently increasing in width during ontogeny; sutures circular, depressed, not very clear; aperture terminal, produced on a very low neck with an everted lip, lip marked by irregular grooves; wall calcitic, hyaline, perforate, pores restricted to the low ornamentational ridges, ornamented by annular ridges, annuli connected by a single longitudinal ridge, running in a fold, number of annuli per chamber increases during ontogeny. Remarks. Differs from M . tessellata in the annular ornamentation rather than a tessellated pattern and in being more flaring. The test is generally also more straight rather than arcuate. As is the case for M . tessellata, a clear dimorphism is present in this species and it is identical to that described for M . tessellata, except that the longitudinal furrow does not occur in M . 1 im ba ta.

Millettia limbata
Millettia polyxenae sp. nov. (Pl. 2, figs 10-15) Holotype. 1958:8:9:15:677, British Museum (Natural History), London. Type locality. Raine Island, Challenger Station 185. Material examined. The holotype, also BM(NH) ZF 4956, Timor Sea, off Java Description. Test free, elongate, elliptical in endview, longitudinal axis arcuate, chambers in rectilinear series except the first ones, periphery clearly lobulate; chambers compressed laterally, broadly ovate, becoming more elongate throughout ontogeny; sutures circular, depressed; aperture terminal, produced on a short thick neck, neck ill delimited, with a broad, slightly everted lip, aperture grooved; wall calcitic, hyaline, pores concentrated along a central band running over the consecutive chambers, ornamented by barely visible annular ridges connected by a longitudinal band. Remarks. Differs from the other species in possessing large, laterally flattened chambers, and inbeing distinctly constricted at the sutures. Because of the very low nature of the ornamentation, it is uncharacteristically smooth for the genus.

DISCUSSION
Despite recurrent attempts to understand the genus Millettia, it has been eluding systematists ever since its original description. The vacillations of the genus in the different classifications proposed over the years clearly indicate the difficulties it has been causing. The observations reported here on the ultrastructure underlying the complex internal morphology only confirm earlier experienced ungainliness and do little to alleviate the problematic status of the taxon.
Lectotype, BM(NH) ZF 4957. Fig. 5.Habitus (150pm); Fig. 6. Oblique view, showing the annular ornamentation (150pm); Fig.7. Close-up of the aperture. Note its irregular nature and the absence of a clearly defined neck (25pm). Topotype, BM(NH) ZF 4958; Fig. 8. Habitus (150pm); Fig.  9. Oblique view, stressing the annular ornamentation (100pm). Revets test, the rectilinear arrangement of the chambers, the aperture produced on a neck and the presence of toothplate-like structures in Millettiu does point to a buliminid parentage. However, the subdivision of the individual chambers by septula, the presence of septular foramina linked to the toothplates and the extension of the septula onto the lateral chamber wall in a web-like fashion are without counterpart in the buliminids (Pl. 1, figs 10-13).
The use of etched sections results in even more surprises. Although the test wall is basically bilamellar, secondary lamination is absent (Pl. 4, figs 7,8). An important part of the test is actually trilamellar due to the deposition of an extra layer of inner lining inside the chamber lumen (Pl. 4,figs 2,4,8). The construction of the septula leads to a partial doubling of the inner lining on the lateral chamber walls, and to a doubled inner lining which form the actual septula (Pl. 4, figs 2,4). Doubled-up inner linings are known only in Elphidiellu and Asterigerinu (see Hansen &Reiss, 1972 andHansen &Lykke Andersen, 1976). Contrary to these genera, the features in Millettiu linked to the double inner lining do not serve to isolate part of the chamber lumen: there is no analogy to the morphological elements in either Elphidiellu (i.e. subsutural canal) or Asterigerina (stellare chamberlets). Theultrastructure of the wall is also highly unusual. Millettiu appears optically radial under polarised light, and this observation is confirmed by the actual disposition of the individual crystal elements. What is unusual is the apparent inability of the organism to bend round corners when depositing chamber wall: the elongated crystals either form twins or leave a discontinuity which shows up prominently in the etched sections. This is very well illustrated in theconstructionof theaperture and the test wall in the immediate surroundings. This peculiar feature enhances the unusual construction pattern of the successive 'chamberlets' (Pl. 4,figs 2,3,4,8).
The detailed mapping of etched specimens gives some indication as to the order in which the test may be built. Apparently, a chamber is constructed a 'chamberlet' at a time. Tracing the different layers, it appears that a break is present in the outer lamella as well as in the inner lining between each 'chamberlet': younger 'chamberlets' seem to rest on top of the previous one (see P1.4).
Also, resorption of the internal partitions seems to occur quite often, leaving the lumina completely empty (Pl. 3 fig. 1). The stripping away of inner lining seems to extend even to the inner lining belonging to the lateral chamber wall, leaving only outer lamella in some places (Pl. 3, fig. 2,4). Contrary to Buliminoides, in which considerable resorption takes place as well, the aperture in Millettiu is not modified in the process (compare P1. 1, figs 8 & 9 with Revets, 1989, P1.5, fig. 5). As is the case for the former, resorption may be linked to reproduction, but at the present time not enough information nor specimens are available to substantiate this hypothesis. Extensive resorption, together with the occurrence of porefields, (i.e. the pores are clustered on the raised anastomosing networkoverlying the outer test walland absent from the 'windows' in between) immediately suggests an affinity to aragonitic taxa (see Schneidermann, 1969 andHansen, 1979). However, X-ray diffraction shows beyond doubt the calcitic nature of Millettia.
Asaresult,Millettia shows some similarities with respectively Robertinid (porefields, resorption, peculiar deposition of CaCO,), Buliminid (Seriality, aperture and presence of toothplates) and Nodosariid (Absence of secondary lamination) taxa. Because of this unique combination of characteristics, Millettiu cannot be placed in any of these. I therefore propose to retain the genus in its own family and to consider the Millettiidae incertue sedis within the Rotaliina. Because of the current ongoing revisions of the toothplate-bearing taxa, it is deemed premature to recognise a separate superfamily for this enigmatic genus.
Despite the fact that relatively few specimens were available for study, an evolutionary scheme within the genus can tentatively be put forward (Fig. 2). The morphological features of M. ipsithillue clearly indicate it as being further removed from the other species. Its very slender nature and quite deviating ornamentation support this claim. Nevertheless, the aperture and the partial occurrence of the hexagonal sculpturing shows its relation to M . tessellutu. M. lirnbutu stands also a bit apart, mainly because of its rather dumpy nature and the different aperture. Contrary to other species, M . limbuta has a non-hexagonal ornamentation pattern and an aperture that is not really clearly produced on a neck. Also, the delimitation of the apertural opening is much more irregular. As a result, it is an unlikely ancestor for the genus. Since only M . tessellutu and M . limbuta have a fossil record, the most likely candidate forstemspecies of the genusseems tobeM. tessellatu. The real difficult resides with the position of M. polyxenue. Because it combines features of the three other species, it may well be a case of parallelism or the beginning of iterative evolution. The finding of a specimen of M . limbutu from Pliocene deposits of Japan seems to point to a geological history of unsuspected interest and diversity. Unfortunately, not enough sightings of these species haveoccurred to resolve the many tantalising questions.