“Mactra fuegiensis, n. sp. Fig. VI.
Testa trigono-ovata, inæquilateralis, tenuis, alba, periostraco flavescente induta, concentric tenuiter striata, carina ab umbone ad marginem posticum instructa; margo dorsi posticus arcuatim declivis, anticus æque descendens sed fere rectus; latus anticum acute rotundatum, posticum paulo latius; margo ventralis late curvatus vel in medio subrectus; umbones approximate, ad apicem leviter erosi; pagina interna alba, cretacea; cicatrices et linea pallii nitentes; cicatrix antica magna, piriformis, postica major, latior; sinus linguæformis, subprofundus.
Long. 51.5, alt. 37.5, diam. 21 mm.
Hab. – San Sebastian Beach.
This species of Mactra appears to be different from any of the known forms from South Patagonia, namely, Mactra edulis, King; exalbida, Gray; Patagonica, d’Orbigny; marcida, Gould; levicardo, Smith; and Jousseaumi, Mabille & Rochebrune. It is rather like the last-named species as figured by Pilsbry (Amer. Journ. Sci., 1899, vol. vii, pl. i, figs. 1-3), but the umbones are less prominent, the anterior end is more pointed, and the size is smaller. The epidermal carina down the posterior side also tends to distinguish it. Mabille & Rochebrune describe their shell as “solidula” and “intus roseo-alba,” whereas the present species is light and thin, and dirty-whitish within.
The hinge is normal, but the lateral teeth are small and rather close up to the cardinals. The small external or marginal ligament is divided off from the resilium by the continuation of the single posterior lateral tooth of the left valve and the lower of the two of the right valve. The concentric sculpture upon the umbones, in fresh, unworn shells, is regular and thread-like, becoming finer as the shell increases. The epidermis is very finely lamellated, and somewhat wrinkled down the posterior side behind the radiating ridge. A second faint ridge is more or less evident down this part of the valves.”
(Smith, 1905: 377)
