RHYNCHONELLID^E 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 a& 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. 



Rh^TicIionella (Stenocisma) contracta. 



PLATE LV. 



Atrypa contracta : IIall, Report on the Fourth Geol. District of New-Tork. Illustration Ixvi, 



flg.3o. 1843. 

 Not Rhynchonella contracta of D'Orbiobt. 



Atrypa laticotta and var. as aborc : Illustration Ixvi, flgs. 1 & 2. 



Kot A. laticotta of Pniiiips, 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. 



