101^ PALEONTOLOGY OF NEW YORK. 



lim- altout ;-)5 mm. ' A smnlli'r s])ocimen is 2(i nun. in k'ugtli, 25 mm. in 

 liciiilit. liiiiuv-liiic 2 1 mill. 



Tliis species l)ears a general re.semblance to P. Chemungensis ; it ditiers in 

 its continuous and comparatively stronger rigid radii. The group oi" four species, 

 beginning with P. Chemungensis, is well marked and have many characters in 

 common; Imt nevertheless, present diilerences which warrant their specific 

 separation. 



Formation and localities. In the (Jheuiuiig group, at several localities in 

 Chenumg county, N. Y. 



Ptkrinea prora. 



I'l.ATK XVI, KICiS. 4, 13, 14. 



PteriHin jimia, IIai.i.. Pal. N. \.. vol. v, i)t. 1. Plat«.< iiiid Kxplaiiatidiis : I'l. U',. tiys. 4, Ki. U. 



Jan., 1883. 



Siii'.i.i, ol' iiu'iliiiiii size or larger, sul)-rhomboidal, oblique; body o\ati', narrow 

 above; length one-tburth greater than the height ; inargiiis I'eguiarly rouiuh'd, 

 extended on the jwst-basal poitioii and sloping i-apidly into tlie .'<iniis ol' the 

 wing. 



fjeft valve moderately convex. Uight \alve depressed-convex aliovi', flat 

 or concave below, apparently smaller than the lel't. 



Hinge-lino straight, longer than the length of the valve, and extending 

 beyond both margins. 



Beak small, acute, directed forward, situated at about the anterior liftii of 

 tlie hinge. Unibonal region undefined, not prominent. 



Ear about one-fil'tli tlie length of the wing, se2)ai'ated from tlie bodvofthe 

 \al\e iiy a sliallo\A undefined sulcus and marked b\ssal sinus; margin con- 

 vex; extremity rounded. Wing large, triangular, extended, defined onl\ bv 

 the change in the curvature of its margin from that of the boilyof the \al\e; 

 margin deeply concave; extremity acute. 



Surface of left valve marke(| with regular, rounded, ''sub-ecpial rays, with 



