CLINTON GROUP. 



81 



We perceive here a regular gradation of form in the ventral valves, from 1 a through 1 d, 

 1 /" and 1 h; and in the dorsal valve, through 1 i, 1 r, 1 e and 1 g, giving the principal va- 

 rieties observed. 



Fig. 1 i. A profile view of a small perfect specimen. 



Fig. 1 k. The interior of two fragments of dorsal valves, showing that the single plate is in 

 effect composed of double laminae, produced by the folding inwards of the inner 

 lamina of the shell. 



Fig. 1 I, m. Dorsal and ventral view of a silicified cast of this species, showing in a very satis- 

 factory manner the internal structure. The specimen is from. Iowa, but resembles in 

 form those of New-York, and is not more gibbous than the majority of specimens 

 from this State. 



Plate XXVI. 



Fig. 1 a, b. A cast : views of the dorsal and ventral valves of a very large a. id gibbous spe- 

 cimen. 

 Fig. 1 c. Cardinal view of the same, showing casts of the interlaminar spaces. 

 Fig. 1 d. Profile view of the same, showing strong concentric ridges of growth. 



Position and localities. This species is very abundant in the Clinton group of New-York, 

 appearing as casts or fragments of shells in the eastern and central part of the State, but be- 

 coming more numerous and perfect in Wayne county, where it is distributed through several 

 feet in thickness of an impure limestone. At Rochester it occupies a thin layer of limestone 

 which is almost entirely composed of this shell, packed together as if drifted upon a beach, 

 the greater portion consisting of separated valves lying one within another. Westward of this 

 point it is scarcely known within the State of New-York, but appears again in Ohio at several 

 places, and is abundant at Milwaukie and the neighborhood of that place in Wisconsin, pro- 

 bably occurring along the northern outcrop of these formations entirely across the country from 

 Lake Michigan to the Mississippi river. It occurs in Iowa and farther to the northwest on the 

 tributaries of the Mississippi. {State Collection.) 



471. 2. PENTAMERUS FORNICATUS (n. 5;).). 



PL. XXIV. Fig. 7 a, b, c, d. 



Helmet-shaped ; length and breadth nearly equal ; dorsal valve extremely convex, arched 

 and incurved over the ventral valve ; beak very prominent, and abruptly incurved ; ventral 

 valve nearly circular, moderately convex ; surface, particularly of the dorsal valve, marked by 

 obsolete longitudinal plications and fine concentric striae. 



All the specimens of this species which have been obtained are more or less imperfect ; and 

 the one figured has the ventral valve crushed, and the shell removed from the beak of the dorsal 

 valve. The dorsal valve usually presents about five or six plications, which are sometimes 

 [ Paljeontology — Vol. n.] 11 



