196 PALEONTOLOGY OF NEW- YORK. 



indistinct groove : beak often considerably elevated and slightly incli- 

 ned over the hinge-line ; area narrow except in the centre, where it 

 perceptibly widens. 

 SuKFACE marked by from six to ten strong rounded ribs on each side of 

 the mesial fold and sinus ; the entire surface with undulating concentric 

 striae, which, towards the front, become strong zigzag imbricating 

 lines. 



The interior of the ventral valve presents a well-defined oval muscular 

 impression with a low crest in the centre. The dental plates are often 

 much thickened, filling the entire rostral cavity and encroaching upon 

 the muscular area. 



The width of the species ranges from one-half to seven-eighths of an inch, and 

 the length is sometimes a little greater but usually a little less than the width. In 

 the more gibbous specimens, the beak of the ventral valve is so extremely eleva- 

 ted that one-half the length of the valve is above the cardinal line. In the majority 

 of specimens, there are about six or seven plications on each side of the valve. 

 The variable gibbosity of the shell gives an apparent variation in the height of the 

 area, the beaks of the two valves sometimes approaching close to each other. 



Some silicitied casts of specimens from Indiana, which I have referred with hesi- 

 tation to this species, have a very sjTnmetrical form : the beak of the ventral valve 

 is moderately elevated ; while the mesial fold is marked bj^ a distinct longitudinal 

 groove', and there is a distinct angular elevation in the bottom of the sinus. The 

 specimens are extremely abundant in a mass of rock associated with Strophodonta 

 perplana ; and in their mode of occurrence have all the appearance of the species 

 as it is found near Louisville, Kentucky. 



Geological formation and locality. This species occms rarely in Upper Hel- 

 derberg limestone, in Eastern New York, and becomes common in Genesee and 

 Erie counties. It occurs at several localities in Ohio, and at the Falls of the Ohio, 

 both in Indiana and Kentucky. This species has been found in Canada West, and 

 has been described by Mr. Buxmos among the Devonian fossils from that region. 



