196 
PALAEONTOLOGY OP 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. 
Surface 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 silicified casts of specimens from Indiana, which I have referred with hesi¬ 
tation to this species, have a very symmetrical form : the beak of the ventral valve 
is moderately elevated ; while the mesial fold is marked by 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 
per plana ; 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 occurs 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. Billings among the Devonian fossils from that regipn. 
