Amphithyris hallettensis Foster 1974

Amphithyris hallettensis Foster, 1974

(Fig. 2, Table 2)

A. hallettensis Foster, 1974: p. 84, plate 7, fig. 20; Plate 15, figs. 1, 2; text fig. 3, part 16; text fig. 24 D–F (original description)— Campbell & Fleming 1981: p. 148— Zezina 1985: p. 199— Hiller 1994: p. 785— Logan 2007: p. 3108— Bitner 2006: p. 29 (comparison with A. buckmani); 2008: p. 442— MacKinnon et al. 2008: p. 322, fig. 23A–C

Type material. Holotype: USNM I 550094A, Paratypes: USNM I 550094B, 550094C, USNM I 550095A, USNM I 550094-550097

Type locality. Antarctica: Ross Sea, off Cape Hallett; South Orkney Islands.

Material examined. Holotype USNM I 550094A, Paratypes USNM I 550094B, USNM I 550094C, USNM I 550095, USNM I 550095A

Diagnosis. Shell ventribiconvex, small dorsal septum, imprints of capillae visible in ventral valve interior. Description. Shell ventribiconvex, transparent with ventral valve more inflated than dorsal valve. Dorsal valve with small septum mid-valve or below (Fig. 2 A and B). Short socket ridges (Fig. 2 C). Ventral valve with imprints of capillae visible interiorly (Fig. 2 D), capillae not visible exteriorly. Hinge teeth not parallel to hinge line, hinge line almost straight (Fig. 2 E). No muscle impressions visible. Exterior surface smooth with concentric growth lines, endopunctae visible (SEM: back-scatter detector) (Fig. 2 F). Beak pointed (Fig. 2 F).

Remarks. In contrast to our observations of (a) a ventribiconvex shell, (b) the interiorly visible imprints of capillae in the ventral valve, and (c) the presence of narrow deltidial plates, Foster originally described the shells as planoconvex or concavoconvex with capillae absent. A. hallettensis differs from A. buckmani, A. seminula, and A. parva in having a ventribiconvex shell, imprints of capillae on the interior of the ventral valve with no visible traces of capillae on the exterior side (as in A. buckmani and A. parva).

Amphithyris richardsonae Campbell & Fleming, 1981: p. 149, figs. 12–20, 23 (original description)— Richardson 1981: p. 143— Zezina 1985: p. 199— Dawson 1991: p. 433— Hiller 1994: p. 784, figs. 10–11 (as A. cf. richardsonae)— Bitner 2006: p. 29 (comparison with A. buckmani), 2008: p. 442— Logan 2007: p. 3108— MacKinnon et al. 2008: p. 322— MacFarlan et al. 2009: p. 262— Lee et al. 2010: p. 180

Type material. Holotype: NZGS Br 2277

Type locality. New Zealand, South Island: Upper end of the Narrows, Long Sound, Preservation Inlet, medium to fine shell sand, depth: 74m.

Material examined. Holotype NZGS Br 2277, ventral valve.

Diagnosis. Wide, oval shell, with correspondingly wide foramen. Dorsal valve flat and closely adherent over its whole area to the substrate (Campbell & Fleming 1981). Foramen amphithyrid, with an obtusely triangular delthyrium, no deltidial plates visible.

Description. As the dorsal valve is lost (M. Terezow, GNS Science, pers. comm.), the description is only based on the re-examination of the preserved ventral valve.

Ventral valve wider than long, exterior with concentric growth lines (Fig. 3 A and B), faint capillae (Fig. 3 A), and very few, randomly distributed tubercles (Fig. 3 B), termed "hollow spinules" by Campbell & Fleming (1981). Ventral valve interior with faint muscle scars at the midline of the umbonal region, septum absent. Teeth small (Fig. 3 C), not parallel to the straight hinge line.

Remarks. The original description of A. richardsonae is based only on the holotype, the interior of which was not exposed by Campbell & Fleming (1981). According to M. Terezow (GNS Science, New Zealand) the holotype was on loan in the past, during which the dorsal valve was lost. The ventral valve shows signs of too harsh a bleaching process to remove the organics and separate the valves. The bleach not only dissolved the soft tissue but also the organic components in the secondary shell layer to the effect that the holotype is now very fragile and fibres of the secondary shell layer disintegrate very easily. We used environmental scanning in our SEM to get some back scatter images of the remaining ventral valve. Comparing the valve's actual outline with the original pictures in Campbell & Fleming (1981, fig. 14) shows that the ventral valve had already lost its posterior lateral portions on either side of the shell due to handling the valve after previous chemical treatment. Campbell & Fleming (1981) distinguished their new species A. richardsonae from A. buckmani and A. hallettensis by its wide, oval shell resulting in a wider foramen and a long, straight hinge line as well as a higher density of punctae and a smaller delthyrium due to its obtusely triangular shape. The comparison with A. buckmani was based on "topotypic juveniles" collected in August 1959 at the Outer Island Bay Bank, Cook Strait, New Zealand (Campbell & Fleming, 1981: p. 146, figs. 1–11). These specimens have been assigned to a new species, A. parva, by McKinnon et al. (2008).

In having randomly distributed tubercles or "hollow spinules" (Campbell & Fleming 1981) on its ventral valve A. richardsonae resembles Neoaemula vector, a new genus and species of Platidiinae described from the fiords of the South Island, New Zealand in McKinnon et al. (2008). Additionally, the outline of the ventral valve of both species is rather similar. Since the dorsal valve of the holotype of A. richardsonae is lost, it is impossible to assess, whether this specimen had a T-bar shaped dorsal median septum—the main diagnostic character of N. vector. Thus, synonymy of A. richardsonae and N. vector cannot be excluded beyond doubt and therefore the taxonomic status of A. richardsonae should be regarded as uncertain (Jeffrey Robinson, pers. comm.).