LAMELLIBRANCHIATA. 425 



Valves moderately convex below, becoming gibbous in the upper and 

 umbonal regions. 



Beaks at about the anterior third, prominent and incurved. 



Surface marked by closely arranged, undulating, concentric striaB, and by 

 about forty strong elevated sub-angular radii, which are narrower than the 

 interspaces. Several of the rays on the posterior slope are bifurcating. 



The two specimens described measure respectively 62 and 63 mm. in 

 length, and 57 and 60 mm. in height. 



This species resembles P. dichotoma (which is not well represented in fig. 21, 

 plate Ixx), but is distinguished by its more elevated and more angular plications, 

 a few of which only are bifurcate on the posterior slope, while in that species 

 they frequently bifurcate or trifurcate. 



Formation and locality. In the Portage group, in the Genesee valley, below 



Portage, N. Y. 



In the Proceedings of the Academy of Natural Sciences of Philadelphia, 

 vol. iii, 1846, p. 23, Mr. Conrad has described two species referable to the 

 genus Panenka. They were described under the genus Monotis, and the pre- 

 viously described species, Pterinea radians, was there referred by Mr. Conrad to 

 the same genus. Panenka radians (Pterinea radians, Conrad), coming from the 

 Hamilton shales, in the State of New York, has been recognized and is described 

 and illustrated in the present volume, vide ante. 



The following are the original descriptions as given by Mr. Conrad : 

 Panenka Poulsoni (Monotis Poulsoni, Conrad). 



" Sub-orbicular, ventricose, not oblique, ribs about 44 in number, obtusely 

 rounded, interstices nearly flat, about as wide as the ribs, with minute trans- 

 verse wrinkles ; umbo broad and the summit prominent ; anterior and poste- 

 rior margins rounded. 



" Locality. Jersey shore, Lycoming county, Pennsylvania. (Devonian shale.) 



" This beautiful species occurs in a dark shale of the same geological age and 

 appearance as the shale of the Chemung Narrows in New York, which is a part 

 of the Devonian System." 

 Panenka elevata (Monotis elevata, Conrad). 



" Obliquely oval, somewhat ventricose, ribs about 42 in number, prominent, 

 acutely rounded, interstices very narrow, except towards the anterior hinge 

 margin, where the ribs are larger; anterior and posterior margins nearly 

 straight ; anterior side very short. 



" Locality. Occurs with the preceding species in the same rock." 



