28 
PALAEONTOLOGY OP NEW-YORK. 
This species is found adhering to valves of Athyris , Tropidoleptus, Strophodonta , 
Spirifer, Avicula , Orthoceras, etc., and separated valves are also free in the shales. 
The form of the shell is very much 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 apes excentric on the posterior side. Some speci¬ 
mens are irregularly subcircular, with 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 
Aulopora- 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 crenistriata. 
PLATE III. 
Crania crenistriata : Hall, Thirteenth Report on the State Cabinet, p. 78. 
Dorsal 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 striae gives the fossil, when partially obscured by 
