318 TAL^ONTOLOGY OF NEW-YORK. 



base affixed just above the middle of the slicll, the point extemlitif^ above the dorsal irargin ; 

 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 forms 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 seeii 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 Cythcrinffi 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. IS. An enlargement of a 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 Lockpbrt : usually covering 

 the surfaces of thin calcareous layers in the shale. 



