SPIRIFER^ OF THE UPPER HELDERBERG GROUP. 215 



about half the entire width of the shell : foramen often limited by a 

 sharp elevated border, which appears to be a projection of the dental 

 plates. 



Dorsal valve gibbous, regularly convex on the sides, a little flattened at 



" the cardinal extremities ; mesial fold abruptly elevated in the lower 



part, often but little elevated or scarcely defined in the upper part ; 



beak small, slightly arched over the sublinear area, which is somewhat 



concave. 



Surface marked by from three or four to eight or nine low, rounded^ 

 often obscure plications on each side : these are crossed by imbrica- 

 ting lamellose striae, which are sometimes wide or distant, and often 

 crowded. The concentric striae are studded with elongated nodes or 

 tubercles, which are thus arranged in parallel bands, more or less con- 

 tiguous, according to the distance of the concentric stiae. 

 The elongate tubercles may perhaps more properly be regarded as 



interrupted radiating striae, which, in the perfect condition of the shell, 



have doubtless extended in slender spines or setae. (They are termed by 



Mr. Conrad short longitudinal striae.) 

 The area is strongly striated vertically. 



It has been impossible to obtain the means of a satisfactory study of the interiors 

 of these shells. A cast of the ventral valve from the Schoharie grit has a deep and 

 subangular sinus and the costse rather strongly marked, with a subovate muscular 

 area which is depressed in the centre with linear elongate occlusor imprints, and 

 cavities at the sides showing short strong dental plates. 



In another cast from the Hamilton group the muscular area is narrower, the 

 dental plates longer and thinner, with faintly marked ribs, and there is evidence 

 of a distinct median septum. The surface of the cast in both valves is strongly 

 marked by vascular impressions. 



This species begins its existence (so far as we know) in the Oriskany sandstone, 

 where it has been rarely observed. It occurs in the Schoharie grit, and reaches it* 

 greatest dimensions in the Cornifcrous limestone ; having often a width of nearly or 

 quite two inches, with a length of an inch and a quarter. It is often well preserved 

 in the Hamilton group ; but in this formation, it rarely attains a width of an inch 

 and a half It is not known in the Chemung group, though there is a representa- 



