32 [Assembly 



Genus Leptoccelia ( Hall, 1856). 



[ Gr. XsTttog, tenuis ; xoiha, venter, in allusion to the shallow visceral 



cavity.] 



Terehratula and Rhynchonella, in part, of authors. 



Atrypa : Conrad, Hall, et al. 



Leptoccelia : Hall, Regent's Report for 1856. 



Shell inequivalved, variable in form, usually semioval or subcircu- 

 lar, transverse or elongate, plano-convex pr concavo-convex : 

 liinge-line sometimes equal to the greatest width of the' shell. 

 /Ventral valve convex or subangular in the middle, with beak 

 more or less extended, moderately incurved; foramen terminal, 

 the lower side formed by two deltidial pieces. Dorsal valve flat 

 or concave, or depresso-convex. A mesial fold and sinus usually 

 existing, but not often prominent. 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 ex- 

 cavated in the base of a strong cardinal process : teeth converging, 

 denticulate, with corresponding denticulations in the sockets. 

 Muscular impressions marking a large oval 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 fossetsj 

 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 mus- 

 cular impression. The muscular impresion 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; and on the 

 centre of the cardinal side of the disc there is a slender process 

 extending downwards, while near the junction of the crura with 

 the disc there is, on each side, a slender descending process con- 

 tinued into the cavity of the ventral valve. The cardinal process, in 

 its central portion, is thickened at first and divided in the middle, 

 but, in old shells, gradually filling the passage to the foramen, and 

 sometimes by a prominent point in the centre entirely dividing the 

 passage. 



The hinge-line is often much extended, and in the dorsal valve 

 nearly straight to the cardinal angles. There is sometimes the ap- 



