138 PALiEONTOLOGY OF NEW-YOKK, 



174. 7. ATRYPA NUCLEUS. 



Pl. XXXUI. Figs. 2 a, b, e. 

 Cpmpare A. cxtans nf supra. 



General form subangiilarly globose ; lengtii and breadth about equal ; cardinal line 

 regularly curved ; beaks small, close pressed, that of the dorsal valve incurving over the 

 other ; margins of the valves uniting very closely and neatly ; mesial sinus broad, flat, 

 extending about two thirds the length of the shell, and produced into a linguiform extension, 

 which is much elevated in front; mesial elevation prominent, rounded, extending about 

 two thirds of the distance to the beak, where it is lost in the general convexity of the shell ; 

 surface of both valves crossed by filiform concentric lines, which are undulated in crossing 

 the mesial sinus and elevation ; no visible longitudinal striae. 



This shell resembles the last one very closely, the principal difference being the extended 

 cardinal line in the former ; the mesial sinus in the latter is flat and more elevated in 

 front, while the mesial elevation on the opposite valve is less extant, rising more gradually 

 from the surface of the shell, and not so distinctly margined by a shallow groove, nor 

 produced in front. 



A comparison of a larger number of specimens may prove the existence of a gradation 

 in the cliaracters, here noticed as distinctive. 



Fig. 1 a. Ventral valve, b. Dorsal valve. «. Profile view. 



Position and locality. MiddleviUe, Herkimer county, in the central portions of the 

 Trenton limestone, associated with Conularia. 



175. S. ATRYPA CUSPIDATAt (n.5j9.). 



Pl. XXXIII.* Figs. 1 a, b, c, d, e,f, g, h. 



Obtusely pyramidal or somewhat obcordate ; width greater than the length ; dorsal valve 

 smaller than the ventral, convex at the sides, with a broad subangular depression which 

 commences near the beak, and becomes broader, deeper and more angular, elevating the 

 front of the shell into a cuspidate extension ; lower half of the sinus marked by a distinct 

 narrow groove in the centre ; ventral valve very gibbous, with a broad mesial lobe which 

 continues nearly to the beak, and extends in front scarcely so far as the line of the shell 

 on each side ; beaks nearly equal, that of the dorsal valve incurved over the ventral valve, 

 the latter being much more gibbous below the beak ; margins of the shell sharp ; surface 

 ornamented with fine elevated concentric striae, which are imbricated towards the margin. 

 The lower half of the shell, particularly on the ventral valve and sinus of the dorsal valve, 

 is marked by numerous sharp radii, which gradually disapj)ear above. 



This species bears a close resemblance to Jltrypa exlans ; Init the specimens of that shell 

 which I have been able to see, are all of separated valves, and differ from this one in Uie 



t PL xxxiii.* is a sriiiplementary plate. 



