RHYNCHONELLID^ OF THE HAMILTON GROUP. 34^ 



Rhynchonella ( 8tenocisma ) prolifica ( n. s.)- 



PLATE LIV. 

 Shell subtriangular-ovate ; length and width about equal : front roun- 

 ded, with a shallow sinus and gentle elevation, depressed in young 

 shells and becoming gibbous in older ones. 

 Ventral valve depressed-convex in the middle, elevated or a little gib- 

 bous on the umbo, curving to the sides and gently depressed in a 

 shallow sinus below the middle : apex slightly incurved. 

 DoKSAL valve equally convex with the ventral, sometimes a little more 

 convex, the difference increasing with age ; regularly convex as far as 

 the middle of the shell, below which the mesial fold becomes more or 

 less elevated, but rarely rises conspicuously above the general outline. 

 Surface marked by about twenty tO twenty-tvo, sometimes twenty-four 

 angular plications, of which three occupy the mesial sinus and four 

 the mesial fold. Sometimes a few imbricating lines of growth are visi- 

 ble, but the finer surface markings are unknown. 

 Length usually less than half an inch, and few individuals exceed this. 

 The casts of the interior show short and rather slender lamellae in 

 the ventral valve, and a short septum in the dorsal valve. 



This species is a very common one iu the eastern part of the State, usually 

 occurring in the condition of casts of the interior. It is distinguished by its slen- 

 der angular or subangular ribs ; the shallow sinus which is gently cuiTed upwards 

 in front, and the nearly straight or little curved beak, while the length and breadth 

 arc usually about equal in young specimens. 



In young specimens there is no perceptible sinus, and sometimes but sixteen 

 plications visible. Some very young specimens from the western part of the State, 

 which may be of this species, have only about twelve plications. Specimens from 

 the Hamilton group of Iowa, received from Mr. O. Sx. John, are of very similar 

 character to those of New- York, but Avith more elongate beak. 



Geological formation and localities. This species occurs abundantly in some 

 compact beds in the shales of the Hamilton group, near Fultonham in Schoharie 

 county ; and is equally or more abundant in some decomposing ferruginous beds 

 near Cooperstown in Otsego county. Small individuals (which I have referred to 

 this species in its young state ) occur at Moscow and York in the Genesee valley, 

 and elsewhere in Western New- York. 



