GENUS PENTAMERUS. 375 



GeXIIS PEiMAMERUS (Sowerby). 



•i 



Types P. knightiiy P, galeaUts, etc. 



Genus Pentamerella (nov. gen.). 



Type p. arata, 



Pentamerella arata. 



PLATE LVIII. 



Alrypa arata : Cos&ad, Ann. Report on the Palaeontology of New-York, page 55. 1841. 



" oetocottata : Id. lb. 

 Pentamerut aratut i Hall, Tenth Report on the State Cabinet, p. 120. 1857. 



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 



