The valves are thin, nearly flat, about half as high as long, broadly gaping. They are rounded, not angular, at the ends of the straight hinge-line. The minute submedian umbones are convex and rounded. Hinge margin very thin but slightly thicker medially, without teeth. Shell entirely and permanently covered by mantle except for a small circular foramen near umbo. A series of cylindrical pointed tentacles arise from the mantle peripherally to the shell margins (emended from McGinty, 1955).

Type species: Aclistothyra atlantica McGinty, 1955 (monotypy).

Remarks

The genus is close to Ephippodonta, differing mainly in the complete absence of hinge teeth. Whether it also should be included in that genus must await further study of the soft parts and interior anatomy.

ACLISTOTHYRA ORIENTALIS SP. NOV.

(FIGS 6I, 51D- F)

Material examined

Holotype: PMBC 20118, reef off PMBC, Thailand, no date, SL = 8.5 mm.

Paratype: Reef off PMBC: no date, one specimen.

Description of material from Phuket

Shell: Smoothly and regularly elongate ovate, very thin and brittle, almost translucent, whitish, slightly shining, surface completely smooth without any sculpturing. Almost flat except for slightly vaulted area near the small umbo, which is located in the middle. Hinge margin nearly straight along the middle third, emarginated anteriorly and posteriorly. Shell margins rounded at both ends. Size of two specimens 8.5 ¥ 4.9 mm and 10.0 ¥ 5.1 mm. The prodissoconch II is large, 400 Mm in diameter. Hinge plate in both valves an elongate, rounded edentulous bar, with an external ligament only, a resilifer being absent.

Soft parts: The valves of one of the specimens were held horizontally; in the other they gaped about 90∞. The shell is overlain by a thin sheet of the reflected mantle except for a small circular foramen in front of the umbones. All along the shell margins the mantle continues as a dorsoventrally flattened girdle peripherally terminating in an edge which sharply divides the mantle into dorsal and ventral parts. Many relatively long cylindrical pointed tentacles issue all along the mantle edge. They number 30–35 on each side and are most crowded anteriorly and posteriorly. Their length varies, probably according to their relative state of retraction.

The dorsal part of the mantle is everywhere beset with numerous regularly spaced wart-like papillae, which are also present in a narrow marginal zone on the mantle’s ventral part. The anterior inhalant region is a vertical slit connecting ventrally with a large pedal gape. A pair of close-set siphonal tentacles guards the dorsal margin of the inhalant aperture. They are similar to the marginal tentacles, but larger. The exhalant opening in both preserved bivalves is small and provided with a single short, thick and pointed posterior siphonal tentacle. Whether the inhalant and exhalant openings may be protruded siphon-like in life as in some species of Ephippodonta is unknown.

Distribution

Phuket Island, Thailand (present study).

Remarks

In several galeommatid genera the valves gape widely and the animals are limpet-like. Shell characteristics, such as shape, thinness, lack of a distinct ornamentation, and especially the complete edentulous condition of the hinge, place the present species in Aclistothyra, but the soft parts cannot be compared, as they are unknown in the only other described species, A. atlantica McGinty. In the likewise limpet-like Ephippodonta the shells are also dorsoventrally flattened.

Species of the subgenera Ephippodonta and Ephippodontoana have a shell ornamentation of distinct papillae; this is developed only toward the margin in the former, while it is arranged along radiating dichotomously branching riblets in the latter. In both subgenera the hinge bears interlocking cardinals and laterals. In the subgenus Ephippodontina the surface of the shell is finely reticulate. Dentition is rather well developed except in E. (Ephippodontina) oedipus Morton in which only questionable laterals are preserved. The only species of Ephippodonta with a smooth shell surface like that of the present species is E. gigas; in the latter, however, the cardinals as well as a resilium have been preserved (Kubo, 1996). In limpet-like galeommatids true marginal tentacles similar to those of A. orientalis, rather than an incised mantle margin, occur in E. gregaria Gofas (assignation to subgenus dubious) and E. (Ephippodontina) murakamii Kuroda, but the tentacles in these species are smaller and more, or much more, numerous.

Etymology

The species is named after the geographical region where it was found.