42© 
PALAEONTOLOGY OF NEW-YORK. 
flattened in the centre : beak little elevated above the hinge-line, and 
scarcely incurved. 
Surface marked by ten to fourteen simple rounded and moderately ele¬ 
vated costae; concentrically marked by delicate imbricating lamellae, 
which are usually almost obsolete or obliterated when the shell is 
silicified, leaving smooth rounded plications. 
This species resembles in general form the S. cumber Ian dice, but is a smaller and 
more delicate shell, with a smoother surface and fewer plications, the space of four 
on the margin of that species covering six or seven on this one. The muscular im¬ 
pression of the ventral valve is proportionally smaller and less distinctly striated, 
while the median septum is scarcely developed below the cavity of the beak. 
Fig. 7 a, b . Ventral and dorsal views of a specimen of this species. 
Fig. 7 c. A specimen with one of the cardinal angles more extended. 
Fig. 7 d. Front view of the preceding. 
Fig. 7 e. Profile view of a specimen of this species. 
Fig. 7 f. The interior of the ventral valve. 
Geological position and locality. In the Oriskany sandstone : Cumberland, Md. 
Spirifer tribulis ( n. s ). 
Plate XCVI. Fig. 8 a - e. 
Shell transverse, varying from semicircular to semielliptical; cardinal 
extremities more or less rounded, gibbous in the middle. Ventral valve 
more convex than the dorsal : beak elevated and incurved ; sinus 
narrow and shallow above, becoming deeper and subangular below; 
area variable, usually of moderate height, the exterior margins some¬ 
times strictly defined. Dorsal valve very convex towards the umbo : 
beak incurved, and often elevated above the hinge-line ; a narrow area. 
Surface marked by from four to six or seven plications on each side of 
the mesial fold and sinus : plications elevated, abruptly rounded, the 
depressions subangular towards the margin ; concentrically marked by 
fine lamellose imbricating striae and finer radiating striae, which cover 
the entire shell. 
