344 PALEONTOLOGY OF NEW-YORK. 



Rli} iicliouella ( Steiiocisiua ) dotis ( n. s.). 



PLATE LIV. 



Shell subtriangularly ovate, wider than high, rounded in front, or some- 

 times truncate in the middle ; a shallow mesial depression and mode- 

 rate elevation in front. 

 The dorsal valve is more or less gibbous, and the plications at the 



sides are curved abruptly to the margin of the ventral valve. 



The plications are subangular or rounded, and about eighteen may be 



counted in the larger individuals. The surface has been marked by fine 



concentric stri£B, and one or two imbricating lines of growth near the 



front. 



In two partial casts the plications are subnodose at the junction of 



these imbricating lines, where they are abruptly incurved on the margins. 

 The largest specimen observed has a width of half an inch, with a 



length of seven-tenths and a depth of one-quarter of an inch. 



This shell resembles the preceding one in its general outlines, being propor- 

 tionally a little wider : the ventral valve is a little more flattened on the sides, 

 the ends of the plications not so much curving to the dorsal valve, and the sinus 

 curving more abruptly in front. The beak is very slightly incuived. 



Geological formation and localities. The most characteristic specimens of this 

 species have been found in a calcareous layer in the upper part of the Hamilton 

 group, at Eighteen-mile creek on Lake Erie ; and others from Geneseo and York 

 in Livingston coimty, New- York. 



Rli}Tichonella ( Stenocisiiia ) carica ( n. s.). 



PLATE LIV. 



Shell round-ovate ; length and width about equal, rounded in front. 

 Ventral valve moderately convex above the middle, curving gently to 

 the lower lateral margins and abruptly to the cardinal slope, regularly 



