130 
PALAEONTOLOGY OF NEW-YORK. 
This species differs from 0. occidentals, in the absence of a depression on the ventral 
valve, as well as in general form, striae, and other particulars. It bears about the same 
relation to the last, that 0. subquadrata does to O. occidentalis. 
The distinctions here indicated are believed to depend upon essential differences of 
structure, which are shown 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 
the sinus of one valve, with the corresponding elevation upon the other, are unduly 
developed. 
Fig 1 a. Ventral valve of a young specimen. 
Fig. 1 b. Profile view of the same. 
Fig. 1 c. Front view of the same. 
Fig. 1 d. Dorsal valve of another specimen, with a deep sinu3. 
Fig. 1 e. Front view of the same. 
Fig. 1 f 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. 1 h. Dorsal valve of an old individual. 
Fig. 1 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 found as¬ 
sociated with well known Trenton limestone fossils, and have been collected by myself, as 
well as subsequently furnished by several friends in Ohio and elsewhere. 
