138 



BRITISH SILURIAN BRACHIOPODA. 



of the dorsal valve of L. fiahellites there exists a large promineDt cardinal process, 

 and on either side the inner socket ridges occupy a lower level. In the same valve 

 of Atrypa ? Jiemispharica, on the contrary, instead of a large cardinal process there 

 exists a small pit or depression, the inner socket-ridges projecting on either side, 

 while the sockets themselves are likewise much smaller, and there is no evidence of 

 the existence of crura or a loop as described to be present in Leptoccelia fiahellites. In 

 A. ? hemisphcBrica a blunt median ridge commences under the pit already described, and 

 extends to about half the length of the valve ; while on either side may be sometimes seen 

 the scars left by the adductor or occlusor muscle. I have also observed differences in the 

 interior of the ventral valve of the shell under description and that of L.flabellites. It 

 will, therefore, I think, be desirable to ascertain whether A. hemisphcBrica was possessed 

 of crural processes and a loop before placing it in the new genus Leptoccelia; and in 

 order to assist the reader in the researches he may feel disposed to make on the subject, 

 I will in a foot-note transcribe the description Prof. Hall has given of his genus.^ His 

 figures are copied at page 137. 



* Genus — Leptoccelia, fl^aZZ. Regents' Report on the State Cabinet, p. 67, 1857, and Pal. Xew York, 

 ^ vol. iii, p. 447, 1859. Type — L. fiahellites, Conrad, sp., Terebratula and 



Rhynchonella, in part, of authors. Atrypa, Conrad, Hall, &c. 

 " Shell 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 extended, moderately incurved : foramen 

 terminal, the lower side formed by two deltidial pieces ; dorsal valve fiat 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 excavated in the base of a strong cardinal process ; teeth converging, denticulate 

 with corresponding denticulations in the sockets. Muscular impressions marking a large oval or flabelli- 

 from 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 either 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 impressions. The muscular impression forms a suboval space divided througl| 

 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 continued into the cavity of the ventral valve. Tiie 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 appearance of a false area on the ventral valve, somewhat similar to Atrypa, 

 the margin being thickened and grooved ; but this does not appear to be a characteristic or constant 

 feature. The specimens of the interior, which have fallen under my observation, usually preserve only the 

 short crural processes ; and it is in one specimen alone, which is partially filled with crystalline matter, 

 that distinct cavities can be seen corresponding with what I have described (see figure p. 137). The 



