ORISKANY SANDSTONE. 
447 
Genus Leptoccelia (Hall, 1856). 
Regents’ Report on the Slate Cabinet of Natural History for 1856, published 1857. 
Generic Description. Inequivalved, variable in form, usually semioval or 
subcircular, transverse or elongate, plano-convex or concavo-convex : 
binge-line sometimes equal to the greatest width of the shell. Ventral 
valve convex or subangular in the middle, with beak more or less ex¬ 
tended, moderately incurved ; foramen terminal, the lower side formed 
by two deltidial pieces. Dorsal valve flat or concave, or depressed- 
convex. A mesial fold and sinus usually existing, but not often promi¬ 
nent. Structure of shell lamellose or fibrous, not punctate. 
Valves articulating by means of two strong teeth in the ventral, inserted 
into sockets in the dorsal valve, which are mainly excavated in the 
base of a strong cardinal process : teeth converging. Muscular im¬ 
pressions marking a large ovate or flabelliform area with a thin median 
septum : adductor imprints small. 
The dorsal valve is marked by a strong cardinal process, at the base of 
which, on each side, are the deep oblique dental fossets; and from the 
inner margins of these proceed the crural processes, supported below by 
thickened plates which extend obliquely for a short distance towards the 
middle of the shell, bordering the muscular impression. The muscular 
impression forms a suboval space, divided through the middle by a low 
median septum. 
The crura, in their extension, are united in a flattened disk, which 
terminates at its remote extremity in an acute point, with a central 
projection pointing upwards ; while at the junction of the crura with the 
disc there is, on each side, a slender process continued downwards into 
the cavity of the ventral valve. The cardinal process, in its central por¬ 
tion, is thickened at first and divided in the middle, but, in old shells, 
gradually filling the passage to the foramen, and sometimes by a promi¬ 
nent point in the centre entirely dividing the passage. 
