499 PALAEONTOLOGY OF NEW-YORK. 



are very strong, but scarcely extended below the limits of the area. The 

 cardinal process in the dorsal valve is not so high as the area, but the 

 crural processes are very strong and prominent, with deep fossets for the 

 reception of the teeth of the opposite valve. 



In general form this shell resembles S. mucronattis, but is conspicuously distinct 

 in its wider area : it is usually broader, with the mesial sinus and elevation nar- 

 rower than in that shell. The mesial elevation on the dorsal valve is often flattened, 

 but sometimes rounded. The concentric lamellose striae are sometimes but faintly 

 developed, or have become obsolete from weathering or the process of silicification. 



Fig. 9 a, b. Dorsal and ventral views of a large individual, showing the foramen almo»t 



entirely closed. 

 Fig. 9 c. Profile of the base or front of the shell. 

 Fig. 9 d. Lateral profile view. 

 Fig. 9 e. Ventral view of a specimen from the upper part of which the shell is removed, 



showing the cast of the muscular area and the rostral cavity. 



Fig. 9/1 Interior of the dorsal valve, showing the cardinal and crural processes, with the 

 dental fossets. 



Fig. 9 g. Interior of the ventral valve, showing the partial closing of the foramen, with a 

 perforation at the summit, the dental lamellsa, etc. 



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



Spirifer arrectus ( n. s.). 



Plate XCVIL Fio. 1 a - A, & 2 a - ». 



Shell transverse, varying from semicircular to semielliptical ; cardinal 

 angles sometimes rounded, but often produced beyond the width of 

 the shell below. Ventral valve more or less gibbous than the opposite : 

 beak elevated and incurved ; the area above the medium height, more 

 or less concave, extending to the hinge extremities, separated from the 

 exterior shell by a sharply defined margin ; foramen large ; sinus 

 varying from a depression of moderate depth with curving sides and 

 base, to a deep angular depression which elevates the mesial portion of 

 the opposite valve in an angular fold. Dorsal valve often very convex 

 in the middle and towards the front ; the mesial fold often abruptly 

 elevated, and varying from a rounded to a sharply angular prominence ; 

 the beak incurved beyond the hinge-line. 



