SS1S 
PALAEONTOLOGY OF NEW-YORK. 
flattened toward the front; margins angular and sharply defined. Area 
large and high, flat and inclined a little backwards or slightly concave; 
foramen large and open to the apex. 
Dorsal valve more or less gibbous and sometimes only moderately con¬ 
vex, curving to the front and lateral margins, and a little flattened at 
the cardinal extremities; mesial fold moderately elevated, strongly 
defined, and flattened or concave on the summit : the beak and adja¬ 
cent portion of the margin is more or less arcuate, and the area is 
concave for more than half its length on each side of the centre. 
The surface is marked by ten or twelve plications on each side of the 
mesial fold and sinus; the plications rounded or subangular, and some¬ 
times subnodose on exfoliation. Portions of the shell preserved on some 
of the specimens show strong lamellose concentric striae, with faint 
radiating striae. 
All the s Decimens which have come under my observation are more or less 
imperfect, and in the condition of partial or entire casts. 
The figures 20, 21 and 22 are from a typical specimen of this species, from 
which the shell has been mainly exfoliated. 
Figures 24 and 25 are of a specimen from near Williamsville, New-York, which 
has similar proportions and the same number of plications ; but these are suban. 
gular and a little nodose, while the mesial fold is more distinctly depressed in the 
centre. 
In some respects this form resembles 8. varicosa ; but though much larger, it 
has fewer plications. 
There is another form much less gibbous and more extended on the hinge-line 
than the two preceding specimens ; of this, a large individual is illustrated in 
figures 26-30. The proportion of area and length of shell are similar, the mesial 
fold is flattened or concave on the summit, and the number of plications is the 
same : it differs only in the lesser gibbosity of the dorsal valve and the greater 
extension of the hinge-line. 
Geological formation and localities . The specimens from which this species 
was originally described were from Ohio, occuring at Sandusky and near Colum¬ 
bus. In New-York, it is found at Williamsville, in the Comiferous limestone. 
