310 PALiEONTOLOGY OF NEW-YORK. 



resemblance to /?. pn'mipilaris or implexa. It is never abundant, and is often found 

 in a crushed and distorted condition. 



Geological fonnations and localities. In the Hamilton group, at Moscow, York, 

 Gcneseo and elsewhere in Western New- York; also in the Marcellus shale at 

 Avon and other places. 



Rliynclionella (Stenocisma) sapplio. 



PLATE LIV. 



Rhynehontlla tappko : Hall, Thirteenth Report on the State Cabinet, p. 87. 1860. . 



Shell, in full-grown individuals, transverse, gibbous, subelliptical. Young 

 shells broadly subtrigonal, becoming short-ovate ; nearly straight in 

 the middle of the front, abruptly pointed at the beak : cardinal slopes 

 concave ; sides rounded to the mesial fold and sinus. 



Ventral valve flabelliform, depressed-convex, rarely a little gibbous on 

 the umbo, flattened and depressed in the middle towards the front ; 

 the sinus becoming perceptible about the middle of the length : sides 

 nearly flat ; apex abruptly acute, and more or less incurved according 

 to age. 



Dorsal valve gibbous, regularly arching transversely ; the mesial fold 

 becoming conspicuous only towards the front : in young shells only 

 moderately convex. 



Surface marked by twenty to twenty-two or twenty-four plications 

 (fourteen or fifteen in the young shells), those towards the cardinal 

 margin less elevated ; about four to six mark the sinus and fold. In 

 old shells, the plications are grooved towards the front ; and those of 

 the sides of the dorsal valve are very abruptly bent towards the ven- 

 tral valve, with a shorter and wider groove, and all deeply bifid for the 

 reception of the opposite plication. The shell is concentrically marked 

 by fine thread-like elevated strias, which are more conspicuous and 

 » strongly undulating towards the front of the shell, their remains being 

 often preserved in the casts. 

 Length and width, in full-grown shells, as seven to eight. 



