On Arcacythere Hornibrook, 1952 (Cytheracea, Ostracoda, Crustacea), a senior synonym of Rockallia Whatley, Frame & Whittaker, 1978

The type specimens of Arcacythere chapmani Hornibrook, 1952 (the type species of Arcacythere Hornibrook, 1953; Tertiary, New Zealand) have been re-examined and are shown to have internal carapace features identical to those of Rockallia Whatley, Frame & Whittaker, 1978. Rockallia is known from Cenozoic deep-sea sediments worldwide and from the Oligocene and Miocene of northwestern Europe. The external carapace morphologies of Arcacythere and Rockallia show only minor distribution of the fossae. Rockallia is, therefore, shown to be a subjective junior synonym of Arcacythere. An emended diagnosis of A. chapmani is given.


INTRODUCTION
The genus Arcacythere was erected by Hornibrook (1952) to accommodate the single cytheracean species A. chapmani Hornibrook, 1952, which he recorded from the late Cretaceous (Piripauan Stage) to the Middle Miocene (Waiauan Stage) of New Zealand. Whatley et al. (1982) noted the similarity ofArcacythere toRockallia Whatley etal., 1978, agenus originally described from Holocene and Recent sediments of the Rockall Trough, North Atlantic, but were unable to synonymise them based on the original description and illustrations of Hornibrook. Specimens of Rockallia were later found throughout the Southwest Pacific in Cenozoic D.S.D.P. core material and in Oligocene and Miocene strataofnorthwesternEurope. Whatley etal. (1982) considered that Arcacythere belonged to the Pectoc ytheridae, based on its original description and illustration, and erected a new family, Rockalliidae, to accomniodate both Rockallia and an undescribed Tertiary genus from Argentinian Patagonia.
I have recently recovered from New Zealand Tertiary shelf sediments specimens similar to Rockallia. This has prompted a reexamination of the type material of Arcacythere and a reassessment of its affinities with Rockallia.

MATERIAL
Material examined includes type specimens of Arcacythere chapmani housed at the New ZealandGeological Survey, Lower Hutt. The original sample from which these specimens were picked was collected by J.S. Marwick In addition 1 have recovered specimens of Arcacythere from the late Eocene Kaiatan and Runagan Stages (Ashley Mudstone Formation of a Waihao River bank outcrop, South Canterbury, sample J40/f90, and Totora Limestone of Taylors Quarry, North Otago, sample 5411 f8244A, respectively) and from the early Miocene Otaian and Altonian Stages (of the coastal outcrop of All Day Bay, North Otago; Gee Greensand Formation, sample J42/f6544 and Rifle Butts Formation, sample J42/f169 respectively). Some specimens from these localities are also illustrated here. Figured specimens have been deposited in the museum collections of the Department of Geology, University of Otago, Dunedin, New Zealand, and are catalogued with numbers prefixed OU.

OBSERVATIONS
Topotypes of A. chapmani were examined using the scanning electron microscope (see Plate 1) and transmitted light (see Fig. 1). These techniques allow illustration of important carapace features in greater detail and accuracy than Hornibrook was able to achieve in his drawings ofArcacythere in 1952. Internal carapace features are shown in PI. 1, Figs 3, 4, 8-1 I, and Fig.1, and are consistent with those of Rockallia (see Whatley et a/. 1978 andWhatley et al. 1982). The most significant features in common include: (1) a median hinge element which lacks crenulation, (2) interdigitating adductor muscle scars, (3) a large and subreniform frontal scar, and (4) a large triangular fulcra1 point which is almost dorsal to the adductor scars. Other internal features suchas the normalandradial porecanalnumber anddistribution are also consistent with the generic diagnosis given by Whatley et al. (1982) for Rockallia.
The overall external morphology, inflation and lateral outline of A . chapmani is also similar to that of Rockallia. The nature of the reticulate ornament is comparable to Rockallia and in some specimens bears weak papillate ridges as it does in some specimens of Rockallia. The two large normal pore conuli near the anterior and posterior margins reported in species ofRockallia are also present in A. chaprnani.
The features listed above clearly indicate that Arcacyrhere and Rockalliu are congeneric. Differences between A. chapmani and species assigned to Rockallia can be seen in details of the external carapace morphology and lateral outline (see emended diagnosis ofA. chapmani below). These features identify the former as a distinct species but are insufficient to merit a generic distinction.  Fig. 1) Emended diagnosis. A species of Arcacythere subrectangular in lateral view often with a distinct postero-ventral angle and almost straight ventral margin. Two ridges, sometimes broad, occur parallel and close to the anterior and postero-dorsal margins; the latter extends anteriorly to mid-length. Reticulation robust with vertical components tending to dominate and, in anterior half of carapace, radiate from mid dorsal region. Remarks: This species is similar to A eocenica (Whatley ef al.) from the Eocene of D.S.D.P. Site 207 in the Tasman Sea, but differs in its robust reticulation and ridges. A. woutersi (Whatley et al.) from the Oligo-Miocene of northwestern Europe differs in lacking the strong anterior and postero-dorsal ridges present in A. chupmuni, but is similar in its robust reticulation, a feature presumably reflecting similar relatively shallow water depths at which these two species occurred. Stratigraphical range. Hornibrook (1952) states that Arcacythere chupmuni ranges from the Piripauan Stage (Late Cretaceous) to the Waiauan Stage (Middle Miocene). New Zealand records older than Oligocene probably involve different species since they lack an anterior extension of the poster-dorsal ridge, have a more convex posterior outline, and a less robust reticulation (see P1. 1, Figs 14-17).