SPIRIFERiE OF THE UPPER HELDERBERG GROUP. 
£13 
• . 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. 
Doksal 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 
sub angular sinus and the costas 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 its 
greatest dimensions in the Corniferous 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- 
