•4 PALiEONTOLOGY OF NEW-YORK. 



sloping to the margin, and flat or concave upon the umbo : hinge 

 extremities often slightly deflected to the dorsal side. 

 Area of the ventral valve from one to two lines in width, and variously 

 inclined to the plane of the lateral margins of the shell, transversely 

 striated and the inner margin crenulated for about half the distance 

 from the centre to the cardinal extremity ; the remaining portion 

 marked by a narrow groove (the latter feature not always distinct). 

 The foramen is closed, a smooth triangular space marking its place, 

 with a narrow callosity along its centre. Area of the dorsal valve 

 narrow and linear throughout. 

 Surface marked by angular subequal interrupted or rugose striae, which 

 are often made to appear more equal and uniform by the partial exfo- 

 liation of the shell. The striae are bifurcated two or three times before 

 reaching the margin, and they are sometimes increased by intercala- 

 tion. As usually preserved, they rise at intervals into sharp ridges 

 or elongate nodes; and in well-preserved specimens, these are pro- 

 duced into short spines.* 



When the shell is exfoliated, the surface between the striae is marked 

 by minute pores or tubular openings, which communicate with pustulose 

 elevations on the interior surface of the shell. In the entire cast, these 

 pores mark the surface with considerable regularity, varying in size, and 

 uniformly larger below the middle where the valve is more abruptly 

 deflected. 



The muscular impressions of the ventral valve are very large and broad, 

 and the margins distinctly limited by a curving elevated border. It is 

 divided in the middle by a more or less developed septum, which termi- 

 nates in a prominent callosity beneath the place of the foramen : this 

 callosity gives a bilobate character to the cast of the rostral cavity. The 

 place of the occlusor muscles is strongly marked, and that of the divari- 

 cator muscles distinctly lobed. The muscular imprints are about two- 

 thirds as long as wide, and the width is often nearly equal to half the 



• This feature is rarely preserved in parts of tlic surface in our specimens ; but Mr. Biilinos has figured 

 « •pecimcn with the entire surface covered with short spines (Canadian Journal, loc. cit.). 



