ORISKANY SANDSTONE. 447 



GeSVS LEPTOCfflLIA (Hall, 1856). 



Regents' Report on the Slate Cabinet of Natural History for 1856, published 1857. 



Geneeic Description. Inequivalved, variable in form, usually semioval or 

 subcircular, transverse or elongate, plano-convex or concavo-convex : 

 hinge-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. 



