•8 PALAEONTOLOGY OF NEW-YORK. 



This species is found adhering to valves of Jlthyris, Tropidoleptus, Strophodonta, 

 Spirifer, ^vicuJa, Orthoceras, etc., and separated valves are also free in the shales. 

 The form of the shell is very mnch influenced by the form of the body to which 

 it adheres. The dorsal valve is often regularly oval and symmetrical, rising to a 

 low subconical form, with the apex excentric on the posterior side. Some speci- 

 mens are irregularly subcircular, witli the surface wrinkled and uneven. 



In a single specimen adhering to a Tropidoleptus, the form of the plications of 

 that shell are carried into the dorsal valve of the Crania, conforming in part to the 

 curvature of its margin as the shell increased in size. The form of the ventral 

 valve is influenced not only by the contour of the body to which it adheres, but 

 by the small bodies growing upon the same surface. In a single example, the 

 central portion of the posterior margin is produced in a narrow extension be- 

 tween two Spirorbides, which prevented the shell from growing in that direction. 

 The dorsal valves are not unfrequently partially covered by an adhering 

 ^tt/opora-like body ; and the small Spirorbis angulatus is likewise found adhering 

 to them. 



In some of the specimens, the form and character of the interior of the ventral 

 valve resembles that of the C. obsoleta of Goldfuss, from the Eifel, which is of 

 Devonian age. 



Geological formation and locality. In the shales of the Hamilton group, in the 

 region of Canandaigua lake and on the shores of Seneca lake ; at Bethany and 

 Covington, Genesee county ; Richmond and Bristol, Ontario county ; and in Erie 

 county. Likewise at Cazenovia and Hamilton in Madison county, and elsewhere 

 in New- York, as well as in the Hamilton group in Maryland and Virginia. 



Crania crcnistiiata. 



PLATE III. 

 Crania erenutriata : IIall, Thirteenth Report on the State Cabinet, p. 78. 



D0B8AL or upper valve very depressed-conical, subcircular ; apex central 

 or subcentral, a little inclined. 



Surface marked by sharp elevated crenulate striae, reaching almost to 

 the apex (which is quite smooth), and increasing by interstitial addi- 

 tions. 



This species is rare, and four specimens only of the dorsal valve are known at 

 this time. The character of the itria; gives the fossil, when partially obscured by 



