Copytus elongatus Benson 1964

Copytus elongatus Benson, 1964.

While C. laevata (senior synonym of C. caligula) is rather thin-shelled, another Antarctic species Copytus elongatus Benson, 1964, is more robust. It is smaller (l = 0.79–0.83 mm) than C. laevata (l = 0.94–1.02 mm), but also has vestibulae in both ends, and few short marginal pore canals, although numerous are false. Its hinge is similarly adont, but more robust, and its outline is rounded anteriorly and posteriorly. The adductor muscles leave an obscure small cluster of scars; the frontal scar is not visible in this species. Apart from its stronger shell and more robust hinge, Benson (1964, p. 17) stated that C. elongatus differs from C. laevata by its “narrower-mouthed, deep anterior vestibule and shallower posterior vestibule.” Neale (1967, p. 28) separated these two species mainly on their length: height ratio. He demonstrated that in his material of C. elongatus and Benson’s type material the height of the specimens varied between about 41 and 43% of the length compared with about 33% for Skogsberg’s C. caligula from South Georgia, 33 to 39% for Brady’s type material of C. laevata from Heard Island and 39% for Chapman’s (1916) Quaternary material from the Drygalski Glacier (Antarctic Peninsula). Neale’s photographs (pl. I, figs q, q’, r, r’) emphasised the very straight and parallel dorsal and ventral margins of C. elongatus. Benson’s species is recorded widely throughout Antarctic waters, the most northerly record being in the South Orkney Islands. Benson (1964) and Neale (1967) record it from McMurdo Sound and Halley Bay, respectively, and Hartmann (1997) from a number of localities including the Bransfield Strait, Elephant Island, King George Island, Adelaide Island and Faraday Station. Hartmann (1997, p. 75) gave a synonymy of the species, which, however, excluded the record by D’Onofrio & Pugliese (1989 apud Hartmann 1997) from the Ross Sea.