!•• PALiEONTOLOGY OF NEW-YORK. 



The interior of the ventral valve is marked by large flabelliform di- 

 Tftricator muscular impressions, extending more than half the len^h of 

 the shell ; narrow above, with sides nearly straight, curving below and 

 deeply divided in front, each division showing four or five lobes. The 

 occlusor impressions are two semioval elevated spots a little below the 

 apex, the centre becoming a thickened ridge or process lying beneath 

 the place of the foramen, with a cavity on each side for the insertion of 

 the bifurcate dorsal cardinal process. The muscular impression is exca- 

 rated in the substance of the shell, the margin in the upper part being 

 elevated and marked by a row of pustules. In young shells the muscular 

 impressions are often indistinctly limited, but in the older shells are 

 very well defined. The muscular impressions of the dorsal valve are dis- 

 tinctly but not strongly marked : these are separated above by a median 

 ridge which divides in the bifurcating cardinal process, and this is 

 supported on each side by an oblique pustulose ridge which gradually 

 iperges into the surface of the shell. 



In general, the shell is readily recognized by ile nearly flat form and 

 fine nearly equal striae. In the muscular impressions of the interior, it 

 resembles 8. beckii of the Lower Helderberg group, but is never so strongly 

 wrinkled. The smoother specimens of that species in limestone are not 

 easily distinguished from the ordinary forms of this one as they occur in 

 the Corniferous limestone. 



The original of S. perplana of Conrad was from the Onondaga limestone of the 

 Upper Helderberg group, a specimen about five-eighths of an inch in width. 



The iS. jduriitriata of the same author was described from an impression of the 

 ventral valve in arenaceous shale ; and similar specimens from the same vicinity 

 as the original show that it does not differ from the S. perplana, while casts of 

 that species occur in the same association. The S.fragilis, Hall, was described 

 from specimens in the calcareous shale of the Hamilton group in Western New- 

 Tork, and from the same horizon at Rock island in Illinois. 



The Strophomena delthyris of Conbad, from the Chemung group, is doubtless a 

 large individual of S. perplana, since I find that similar large casts of this species 

 occur in the same formation. 



