LOWER HELDERBERG ROCKS. 
.197 
and not unfrequently the valves are so closed as to leave no visible area. The beak 
of the dorsal valve, either at or just within its extremity, shows a rounded perfora¬ 
tion or groove; and the beak of the opposite valve is marked by a rounded, shal¬ 
low groove, which, extending downwards, often ends in a perforation both in young 
and old shells. Sometimes the groove only is visible, the perforation having doubt¬ 
less once existed, but subsequently closed. In some specimens there is a simple 
indentation on the beak. 
Fig- la-*. Figures illustrating the usual characters presented by the shells of this species. 
Fig. 1 k (by error marked 1 s). Profile showing the great extent of the deflected portion 
of the shell. Between this one and 1 b +> there are every possible gradation. 
Fig. 2 & 2 +. Specimens which are slightly curved, and showing a larger number of cor¬ 
rugations than usual, which extend nearly to the base of the shell. 
These specimens are from the Upper Pentamerus limestone, and all those ob¬ 
served from this rock preserve a similar character. 
Fig. 1 l, m, n. Interior of the dorsal valves, showing some variety of characters. 
Fig. 1 o, p, q. Casts or moulds left in the stone by the dorsal valves of several individuals. 
Fig. 1 q + . Enlargement of the muscular imprints, etc. 
Fig. 1 r, s, t. Interior of several individuals of the ventral valve. 
Fig. 1 u, x, y. Casts of the interior of the ventral valve fig. 1 u, still retaining a portion 
of the shell. 
Geological positio?i and locality. In the Pentamerus and shaly limestone of the 
Lower Helderberg group, but principally in the latter rock : Helderberg mountains, 
Schoharie, Carlisle, Catskill, Hudson, etc. 
Leptonsi concava (n. s.). 
Plate XVIII. Fig. 2. 
Shell concavo-convex, hemispherical. Ventral valve regularly convex : 
umbonial region prominent; cardinal margin rounding from the beak 
towards the lateral extremities. Dorsal valve deeply concave. Hinge 
line less than the greatest width of the shell. Area of ventral valve 
broad, that of dorsal valve linear. Foramen triangular, nearly closed 
above by a thick callosity, the lower part occupied by the prominent 
cardinal process of the opposite valve. 
Surface marked by very fine close radiating striae, each fifth or sixth 
one a little more prominent than those between; crossed by fine 
regular concentric wrinkles, producing a beautiful subcancellate ap¬ 
pearance. 
