GENUS PENTAMERUS. 
873 
GENUS PENTAMERUS (Sowerby), 
Types P. knigktii, P. galeatus, etc. 
GENUS PeNTAMERELLA (nov. gen.). 
Type P. arata. 
Pentamerella arata. 
PLATE LYIII. 
Atrypa■ arata : Conrad, Ann. Report on the Palaeontology of New-York, page 55. 1841. 
“ octocostata : Id. Ib. 
Pentamerus aratus : Hall, Tenth Report on the State Cabinet, p. 120. 185T. 
Shell ovate, more or less convex or gibbous, becoming arcuate-ovoid in 
old shells; the width greater or less than the length : hinge-line 
variable. 
Ventral valve gibbous and somewhat regularly convex in the young 
shells, becoming ventricose in old shells, with a mesial sinus which is 
more or less developed. In old shells, the form is extremely arcuate 
and the beak strongly incurved; in shells of medium size, the beak 
is obtuse, limiting the apex of the triangular fissure. There is. a nar¬ 
row area bordering the fissure and the space, on either side, between 
the hinge-line and its apex, is often flattened, and sometimes distinctly 
limited by a faint elevation : fissure large, and nearly covered by the 
beak of the opposite valve. 
Dorsal valve, in young shells, more or less convex, and sometimes gib¬ 
bous in the upper part, and often only moderately convex in older 
shells; mesial fold usually well defined in the lower half of the valve, 
sometimes reaching nearly to the apex : in young shells there is 
rarely a short sinus in place of the mesial elevation. 
Surface plicated by rounded or angular plications, which sometimes 
reach nearly or quite to the beak, but are often only developed below 
