378 
PALAEONTOLOGY 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. occidentalis ( = Gyjn'dula occidentalis of Iowa) in 
general form ; but the plications are more numerous, and extend farther towards 
the beak, which is 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 Eavilion in Genesee county, and else¬ 
where in 'Western New-York. 
Peiitamerella micula (n. s.). 
PLATE LYIII. 
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 lamellae 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 expanded than that of P. grata. 
