TRENTON LIMESTONE. 145 



184. 17. ATRYPA '• — {Species undetermined). 



Pl. XXXIII. Figs. 11 a, b, c. 



Somewhat circular or suborbicular ; dorsal valve flat or depressed at the sides, depressed 

 in the centre, with a broad curving sinus which is little elevated in front ; beak small, 

 scarcely incurved ; ventral valve very convex, somewhat gibbous, with a broad medial 

 elevation and sloping sides ; beak small, incurved within that of the dorsal valve; surface 

 marked by filiform concentric lines, without longitudinal plications. 



I have been inclined to regard this species as the young of Jl. extans or A. cuspidata, but 

 there are some objections to thus referring it. The sinus is broad and shallow, not distinctly 

 margined, and is broadly rounded in front, as is also the mesial lobe of the opposite valve. 

 The dorsal valve is flatter, and the beak appears to be less incurved than in either of the 

 species named. It diflers from the last species in the inequality of the valves, and the 

 absence of plications either upon the sinus or sides of the shell. 



Fig. II a, b. Ventral and dorsal valves of this species. 

 Fig. lie. Front view of the sarnie ; the dorsal valve below. 

 Fig. II d. Profile view of the same. 



Position and locality. In the central portions of the Trenton limestone at Middleville, 

 Herkimer county. 



185. 18. ATRYPA SUBTRIGONALIS (»».*j9.). 



Pl. XXXIII. Figs. 12 a, b, c. 



General form subtriangular, with a rounded base ; length and greatest breadth equal ; 

 cardinaJ line short, well defined, with the appearance of a small triangular foramen under 

 the beak of the dorsal valve ; cardinal slopes long, straight ; basal margin regularly 

 curved, with a slight eraargination at the sinus ; valves almost equally convex ; dorsal 

 valve extended beyond the ventral, beak scarcely incurved ; surface marked by about 20 

 rounded radii, three of which are in the mesial sinus and four on the mesial elevation, 

 ornamented by elevated concentric lines which are undulated in crossing the plications. 



A distinguishing character of this shell is its triangular form, with extended small beak ; 

 the mesial sinus and elevation are narrow, and continue only about two thirds of the 

 distance from base to beak. 



This species is rare, occurring only in one or two localities. It may perhaps be regarded 

 as a variety oi Jitrypa increhescens ; but it diflfers considerably in form, and has a larger 

 number of plications, as will be seen on comparison ; even the oldest individuals of fig. 13 

 rarely having more than fifteen plaiu, though specimens of twice the size of this are often 

 found. 



[ Paleontology.] 19 



