202 
PALAEONTOLOGY OF NEW-YORK 
in Schoharie and at Hamilton, Cazenovia and Madison ( Madison county) ; but is 
unknown in this formation in the western part of the State. There is a cast of this 
species in-the cabinet of Marietta College, Ohio, which, I infer from its character, 
has been derived from the southwestern extension of the Hamilton group, perhaps 
in Virginia. 
Spirifera macrothyris. 
PLATE XXX. 
Spirifer macrothyris : Hall, Tenth Report on State Cabinet, p. 132. 1857. 
“ “ Description of New Pal. Fossils, p. 92. 1857. 
Shell large, moderately gibbous, transverse, semielliptical, the width a 
little more or less than twice the length : hinge-line equal to the 
greatest width of the shell; cardinal extremities rounded or angular, 
and produced. Surface plicated. 
Ventral valve less gibbous than the dorsal, regularly arcuate from front 
to beak, and curving gently to the lateral and cardinal margins : sinus 
well defined above, and reaching to the small and neatly incurved 
beak, becoming wider and deeper below and much extended in front. 
Area concave, of moderate width, and extended to the cardinal extre¬ 
mities ; margin angular: fissure very large. 
Dorsal valve the more convex, the greatest convexity about the middle 
of its length : mesial fold prominent, angular in its upper part and 
rounded towards the front, giving a highly arcuate outline to the mar¬ 
gin ; umbo much elevated and rising nearly as high as that of the 
opposite valve; beak incurved over the nearly vertical area, which is 
about a line in width. 
The surface is marked by about ten or twelve strong low rounded pli¬ 
cations on each side, which are simple from their origin, becoming less 
prominent towards the margin of the shell : the first five or six on each 
side of the fold and sinus occupy the greater part of the anterior margin. 
In well preserved specimens the surface is marked by fine closely 
arranged concentric striae, which are finely papillose from radiating 
striae. The cast of the ventral valve presents a large ovate bilobed mus¬ 
cular impression. 
