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PALAEONTOLOGY OF NEW-YORK. 
the posterior margin or less than one-fourth the length of the shell 
therefrom, prominent, being from less than one-eiglith to one-quarter 
of an inch above the plane of the margins. Ventral valve slightly con¬ 
cave ; apex excentric; foramen large, oval, with the margins deeply 
depressed. Shell comparatively thick and strong. 
Surface marked by fine concentric lines, which are very faint in young 
shells, but become stronger and rise into distinct sharply elevated striae 
in older specimens. 
Where partially exfoliated, the shell shows intermediate fine radiating 
striae; and in older specimens, the interior of the shell, and likewise 
the cast, is marked by strong radiating vascular impressions. The 
structure of the shell is strongly lamellose. The casts of the dorsal 
valve show a narrow longitudinal muscular impression on the anterior 
side of the beaks. The length varies from one-eighth to one inch. 
For these specimens, I am indebted to Dr. J. S. Newberry. 
Geological formation and locality. Abundant in a ferruginous band about 110 
feet below the Conglomerate at Cuyahoga falls, and in the green shale and shaly 
sandstone at Akron, Ohio. 
Genus Crania (Retzius). 
Up to the present time, the Genus Crania has not been recognized in the 
New-York collections, from any rock below the Schoharie grit. 
The extensive and long continued explorations in the Lower Helder- 
berg group, which resulted in the discovery of many rare and obscure 
fossils of that formation, we may suppose would have brought to light any 
species of this genus which might have lived in the area of investigation. 
Still we are far from being warranted, by this negative information, in 
the inference that this genus, did not exist in the preceding periods in the 
New-York strata, since We know of its occurrence in rocks of the age of 
the Niagara group in Indiana. At this time we know of but two indivi¬ 
duals in the Schoharie grit, notwithstanding that collections have been 
made from that rock during the past twenty-five or thirty years. 
