496 PALEONTOLOGY OF NEW-TORK. 



flattened in the centre : beak little elevated above the hinge-line, and 

 scarcely incurved. 

 ScRFACE 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. cumberlandia, 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 t. Profile riew of a specimen of this species. 



Fig. If. The interior of the ventral valve. 



Geological position and locality. In the Oriskany sandstone : Cumberland, Md. 



Spirifer tribulls ( n. s.). 



Plate XCVI. Fiq. 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 rety convex towards the umbo : 

 beak incurved, and often elevated above the hinge-line ; a narrow area. 



StJRFACB 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. 



