im 
PALEONTOLOGY OF NEW-YORK. 
The interior of the ventral valve is marked by large flabelliform di- 
varieator muscular impressions, extending more than half the length 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¬ 
vated 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 
merges into the surface of the shell. 
In general, the shell is readily recognized by its nearly flat form and 
fine nearly equal striae. In the muscular impressions of the interior, it 
resembles S. 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 S. pluristriata 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- 
York, and from the same horizon at Rock island in Illinois. 
The Strophomena delthyris of Conrad, 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. 
