SPIRIFER^ OF THE PORTAGE AND CHEMUNG GROUPS, 339 



Spii'ifera Isevis. 



PLATE XXXIX. 

 Delthyrit ItevU : Hali, Geol. Report Fourth District New-York, p. 345, f. 1. 1843. 



Shell ventricose, subcircular or semielliptical, with the cardinal extre- 

 mities rounded : length and breadth as two to three, or three to four ; 

 not plicate. 

 Ventral valve subventricose ; the greatest convexity above the middle, 

 from whence it curves gently to the base and suddenly towards the 

 beak, which is abruptly attenuate and arching over the area : sinus 

 variable, often shallow, gently concave and scarcely defined; sometimes 

 becoming deep, subangular, and very broad in the lower part. Area 

 extending to the hinge-extremities, of moderate height at the sides 

 and rising abruptly towards the middle ; foramen partially closed by 

 an arching, very convex pseudo-deltidium. 

 Dorsal valve depressed or moderately convex, with a wide and usually 

 undefined mesial fold which is much expanded below, leaving the an- 

 terior margin sinuate : sides of the shell somewhat regularly curving, 

 and a little flattened at the cardinal extremities. 

 Surface usually smooth, or marked only by concentric lines of growth. 

 In older shells there are sometimes a few obscure and undefined ra- 

 diating folds. 



The interior of the ventral valve shows strong short teeth with the 

 dental lamellae much thickened, filling up the rostral cavity and ex- 

 tending along the sides of the upper part of the muscular impression • 

 the muscular space is small or of moderate size, deeply marked, and often 

 preserving a distinct median crest. 



In the dorsal valve the muscular impression is small and narrow, and 

 usually not deeply marked. The cardinal process is strongly striated, and 

 this character is distinctly preserved in the cast. 



This species, though occurring in considerable numbers at one locality and less 

 commonly in another, has nevertheless been seen only as imperfect or distorted 



