68 



PALJKONTOLOGY OF NEW-YORK. 



and gradually deepens and widens to the base of the valve, which is nuuh elevated into a 

 linguiform process filling a sinus in the ventral valve ; mesial furrow of the dorsal valve mar- 

 gined by a more or less obtusely carinated fold, produced by a depression in the shell on each 

 side of the sinus ; ventral valve with a strong elevated mesial fold and a depression on each 

 side, beyond which is sometimes an obscure fold ; beak of the dorsal valve very closely 

 curving over the beak of the ventral valve ; entire surface marked by concentric stria', with a 

 few imbricating folds near the base. 



This species is readily recognized in its usual appearance by its rotund and gibbous form. 

 The variations are mainly due to the greater development of the carinas on either side of the 

 mesial fold and depression, which sometimes give the shell a ditferent character, having three 

 prominent folds on the ventral and four on the dorsal valve. This change usually takes place 

 in the older individuals, while the younger ones present only the mesial fold and depression. 

 The specimens usually found are smooth, the striae having been worn or dissolved away ; but 

 in perfect specimens, they appear as fine niised threadlike lines. 



Fig. 1 a. A young specimen. 



Fig. 1 b, c. Dorsal and front view of a young specimen. 



Fig. 1 d, c, f. Ventral view of three different specimens, showing the progressive development 

 in the lateral folds of the ventral valve. 



Fig. 1 g, h. Front view of e ^ /, showing the deep sinus in the ventral valve, and the lingui- 

 form process of the dorsal valve. 



Fig. 1 i. Profile view of a full grown specimen. 



Fig. 1 k, 1. Dorsal valve and cardinal view of a specimen, where the lateral folds are more than 

 usually developed. 



Fig. 1 in. Interior of the dorsal valve, showing the simple dental processes on each side of the 

 foramen. 



Fig. 1 n. A portion of the surface enlarged, showing the striae. 



Position and localities. In the lower part of the Clinton group at Rochester, rarely ; abun- 

 dant at Reynale's basin, and Medina ; and less abundant at Lockport. Also rarely at Flambo- 

 rough head, Canada West. 



454. 25. ATRYPA QUADRICOSTATA (n. sj}.). 



Pl. XXIII. Fig. 2 a, b, c. 



Shell gibbous, suborbicular ; valves sub-equal ; dorsal valve somewhat longer and higher 

 towards the beak, with a deep mesial furrow corresponding to a prominent mesial fold on the 

 ventral valve (this fold, in older shells, has a slight depression along the centre), each valve 

 with two prominent plications on each side of the mesial fold, which reach to the margin or 

 base of the shell ; cardinal area extended on each side of the beak ; surface marked by fine 

 concentric stria;, which undulate on crossing the plications. 



This species very much resembles those specimens of ^3. congesta where the lateral folds 



