SPIRIFERiE OF THE PORTAGE AND CHEMUNG GROUPS. 939 
Spirifera laevis. 
PLATE XXXIX. 
Delthyris Icevis : Hall, 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. 
Yentral 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 
