378 PALEONTOLOGY OF NEW- YORK. 



This species bears much resemblance to P. arata, in some phases of the latter ; 

 but the plications are usually less numerous, rounded and simple, and the shell 

 is more regularly ovate, and never so large and extremely arcuate as in that 

 species. 



The specimens are for the most part in a crushed and distorted condition ; and 

 among a hundred individuals, there is not one preserved in its perfect form. 



This species resembles P. ocddentalis ( = Gypidula occidenlalis of Iowa) in 

 general form ; but the plications are more numerous, and extend farther towards 

 the beak, which ;s more arcuate and less obtuse ; and the ventral valve is marked 

 by a mesial elevation instead of a sinus. 



Geological formation and localities. This species occurs in the shales of the 

 Hamilton group on the shores of Seneca and Canandaigua lakes, at Geneseo, 

 York and Moscow, Livingston county ; at Pavilion in Genesee county, and else- 

 where in Western New- York. t 



PentamercUa luicula (n.s). 



PLATE LVIII. 



Dorsal valve subcircular or transverse, regularly convex. Surface, 

 towards the beak, smooth or marked only by concentric lines of 

 growth ; the lower part of the shell marked by fifteen to twenty short 

 rounded or subangular plications, of which five or six in the middle 

 are slightly more elevated, giving indication of a mesial elevation. 

 Interior of the valve with the lamellsB converging from their origin, 

 and forming a pointed sessile trough, which is contracted where 

 embraced by the teeth of the opposite valve, and expanded near the 

 hinge-line. There is a short low septum just within the apex of the 

 pit, with apparently a narrow area on each side of the fissure. 



Two specimens of this form, from rocks of the age of the Hamilton group near 

 Iowa city, were received several years since ; and the character of the trough or 

 pit in the dorsal valve induced me to regard them as belonging to a distinct 

 genus, but the insufficiency of the material has caused them to be neglected until 

 the present time. The pit is more shallow and expandcid than that of P. arata. 



