130 PAUEONTOLOGT OF NEW-YOBK. 



This species differs from 0. oecidentalis, in the absence of a depression on the ventral 

 valve, as well as in general form, stria;, and other particulars. It bears about the same 

 relation to the last, that 0. suhquadrata does to O. oecidentalis. 



The distinctions here indicated are believed to depend upon essential differences of 

 structure, which are sliown in the three preceding species ; and a further collection of 

 disconnected valves will doubtless enable us to discover additional points of disagreement 

 in specific characters. The three preceding species, with this one, are scarcely regarded as 

 distinct, and are usually found mingled together in collections. This species, and O.sinuata, 

 often suffer distortion from pressure, and on this account are frequently inequilateral, and 

 tlie sinus of one valve, with the corresponding elevation upon the other, are unduly 

 developed. 



Fig 1 a. Ventral vnlve of a young specimen. 



Fig. 1 b. Profile view of the same. 



Fig. 1 c. Front view of the sama 



Fig. 1 d. Dorsal valve of another specimen, with a deep sinus. 



Fig. 1 e. Front view of the same. 



Fig. 1 / Dorsal valve of an older specimen, showing the gibbous ventral valve projecting beyond the 



cardinal line. 

 Fig. 1 g. Front view of the same. 

 Fig. I A. Dorsal valve of an old individual. 

 Fig I i. Front view of the same, showing the deep sinus of the dorsal valve, and the corresponding 



elevation on the opposite valve. 

 Figs. 1 k, m. Cardinal views of young and old individuals. 



Position and locality. In the Blue limestone of Ohio, at Cincinnati and Oxford (Ohio), 

 Maysville ( Kentucky), Madison ( Indiana ) . 



Several of the species of Orthis here given, so far as our present knowledge extends, 

 are exclusively confined to western localities, and may not be found in New-York. There 

 are others which will probably be recognized among the imperfect specimens found in the 

 Trenton limestone of New- York, but which are too imperfect for description. All the 

 specimens figured, except those given on the authority of Mr. Conrad, were foimd as- 

 sociated with well known Trenton limestone fo.ssiis, and have been collected by myself, as 

 well as subsequently furnished by several friends in Ohio and elsewhere. 



