889.] The Genera of Brachiopoda. 985 
muscle scars diverge posteriorly, Obolella Billings. 
{a) Shell structure calcareous (/). 
(/) Dorsal valve limpet-shaped, ribbed, ventral less convex 
iff) Ventral valve usually adhering to 
some foreign object. Inner border 
of both valves rough. Crania Ret. 
Subgenus. 
Free forms, with inner border 
smooth, Pseudocrania McCoy. 
(/) Both valves convex, the larger with a heavy back, large 
area, and pseudodeltidium (^g). 
{g) Shell circular, or transversely oval, area broader than 
long {gg)- 
[gg) Beak of larger valve high and 
heavy, that of the smaller some- 
what truncated, Monomerella Bill. 
iSS) Beak of larger valve low and 
heavy, that of the smaller some- 
what pointed, Dinobolus Hall. 
{£) Shell somewhat elongated, area 
longer than broad, beak of larger 
valve high and heavy, tkat of the 
smaller truncated, Trimerella Bill. 
I. Valves united by a true hinge, shell structure always calcare- 
ous, {h). 
{h) Hinge line curved {hh). 
{hfi) Beak pierced by a foramen, {i)} 
(t) Surface smooth or concentrically striated (u). 
{it) Beak closely incurved, surface 
impunctate. Interior with spires, Athyris McCoy. 
(k) Beak not closely incurved, i.e., a space between 
it and the smaller valve. Surface punctate. No 
spires (/). 
{j) Posterior portion with two 
» Many voune shells have this character, while in adult forms of the same species it is 
