RHYNCHONELLIDiE OF THE CHEMUNG GROUP. 
351 
The shell is usually about three-eighths of an inch in length and breadth, rarely 
attaining the length of half an inch. 
It is abundant in certain layers in a few localities ; but as far as known, it does 
not occur east of the Genesee valley. 
Geological formation and localities. This species occurs in the Chemung group, 
at East-Randolph and Connewango in Cattaraugus county ; at Cherry creek and 
at Howard on Chautauque creek, and more abundantly at Dexterville in Chautau- 
que county, New-York. 
Rliyiichonella (Stenocisma) contracta. 
PLATE LY. 
Mrypa contracta : Hall, Report on the Fourth Geol. District of New-York. Illustration lxvi, 
fig. 3 a. 1843. 
Not Rhynchonella contracta of D’Orbigny. 
Mrypa laticosta and var. as above : Illustration lxvi, figs. 1 & 2. 
Not A. laticosta of Phillips, loc. cit. 
Shell transversely oval or subquadrate; length and width about as seven 
to ten, or.nine to twelve, abruptly and deeply sinuate. 
Ventral valve prominent upon the umbo, with beak abruptly incurved 
and closely appressed against the umbo of the opposite valve, gently 
declining to the lateral and baso-lateral margins, and often nearly flat 
in the last named portions; the centre abruptly depressed into a 
broad' deep sinus, with sloping sides which are free from plications; 
extremely produced in front, with a broad linguiform extension which 
fills a corresponding depression in front of the dorsal valve. 
Dorsal valve somewhat gibbous in the middle, regularly convex trans¬ 
versely ; the mesial fold becoming prominent a little above the mid¬ 
dle, and continuing almost horizontally, or slightly ascending, to the 
front; the sides curving more or less abruptly to the ventral valve, 
according to the convexity of the shell. 
Surface marked by about sixteen angular plications, varying from six- 
. teen to twenty; of which three are usually depressed in the mesial 
sinus, with a corresponding number on the mesial fold. The four pli¬ 
cations of the fold originate as two at the apex, each becoming divided, 
the two central ones being usually stronger than the two lateral ones. 
