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PALEONTOLOGY OF NEW-YORK. 
base affixed just above the middle of the shell, the point extending above the dorsal margin ; 
surface towards the ventral margin depressed ; margin thickened and abruptly inflected. 
This fossil is often abundant on the surface of thin slabs of limestone, and presenting only the 
interior surface as in figs. 17 and 18, it attracts little attention, not being distinguished from the 
ordinary forrfis of Cytherina. A more critical examination usually shows the exterior surface 
of one or two of these, and sometimes we may fortunately detect a fragment where the larger 
number are in this position. But although satisfied from the general form of the apparently 
distinct shells, that they are identical, it is a difficult task to separate them from the adhering 
matrix, and examine the exterior and interior of the same individual. Having done this in 
several instances, there can be no longer any doubt but all those separate valves, showing the 
interior only, are furnished with a spine upon the back. By carefully clearing away the shale 
or calcareous matter along the dorsal margins, the spine may be seen projecting above the line, 
thus proving its existence, when the shell can not be separated from the matrix, and the exterior 
examined. 
The exterior surface, aside from the spine, differs from all the other Cytherinse in the curved 
depression parallel to the ventral margin, and in the thickening of this margin and its abrupt 
inflexion. The interior sometimes shows an indentation at the base of the spine. In the 
character and aspect of its external surface, this fossil has much the appearance of Beyrichia. 
Fig. 17. A fragment of calcareous shale covered with these shells, showing the internal surface. 
Fig. 18. An enlargement of-rn single individual, showing the ordinary appearance of the interior 
under a magnifier. 
Fig. 19. A fragment covered with shells of this species, showing the external surface, with a 
few of those showing the interior. 
Fig. 20. The exterior of a single individual, much enlarged. 
Fig. 21. The interior, showing the spine projecting beyond the dorsal margin. 
Fig. 22. Profile view, showing the thickened ventral margin. 
Position and locality. In the shale of the Niagara group at Lockport: usually covering 
the surfaces of thin calcareous layers in the shale. 
