Description.- External. Shells small, transverse, ventral umbo somewhat prolonged beyond
hinge, angle broad at 100°, posterior walls low, slightly concave in outline. Maximum width at
hinge, with alate convex narrow. Sulcus commences at or a little in front of ventral umbo,
deepens and narrows forward. Dorsal valve concave, curves over short distance into trail but
not geniculate; fold anteriorly placed, narrow with rounded crest. Ventral ornament consists
of about 16 coarse costae which widen than fade anteriorly and converge to some extent
into the sulcal trough. Crossed by some 10 strong rugae over posterior disc, with more in front
laterally, forming coarse undulose reticulate pattern. Rugae curve inwards from the hinge.