«1*8 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 striaj, with faint 

 radiating striae. 



All the 8 jecimens 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 tyi^ical 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, wliich 

 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 S. 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 Corniferoue limestone. 



