«00 PALEONTOLOGY OF NEW- YORK. 



along its inner margins. I have not observed any extensions of dental 

 lamellao into the cavity of the shell. (There appear to be no proper dental 

 plates, but simply a thickening of the margins of the fissure.) Tha mus- 

 cular impression is in the upper part of the valve. 



The interior of the dorsal valve usually presents a quadruple muscular 

 imprint towards the front of the shell, with the bases of the crura at- 

 tached along the inner surface for a considerable distance, and a bilobed 

 cardinal process. 



The casts of the ventral valve sometimes show a median depression 

 in the upper part, and impressed lines slightly limiting the muscular 

 imprints, as if made by a blunt ridge in the shell ; but more frequently 

 there are no tnarks of this kind. 



The small shell in the Marcejlus shale which I originally designated as Orthis 

 nvxleua, is, I conceive, only a smaller form of this species, in which the median 

 sinus is often very conspicuous, being narrow and sharply depressed. It sometimes 

 occurs in the compact calcareous beds of the Hamilton group in great numbers, of 

 a similar diminutive size; and in these the dorsal valve is more convex than usual. 

 In the shales of the Hamilton group this fossil is often extremely abundant, and 

 in a tine state of preservation. The illustrations of the interiors of the valves are 

 of specimens from this group. 



2 



Tim. 1, 2, 8. Dorsal, ventral and profile views ot Jmhocatlia umbonata. natural size, from the shales of 

 the Hamilton group. 

 Fio. 4. The interior of the ventral valve showing foramen, area, etc. enlarged. 



Fio. 6. Interior of the dorsal valve enlarged, showing tlie foveal plates, dental sockets, and the quadri- 

 partite muscular impression; the valve slightly distorted. 

 Fio. 6. A similarly enlarged dorsal valve, showing some variations from the preceding. 



Geological formations and localities. In the Marcelhis shale, at Avon and other 

 places. In the Hamilton group, it occurs everywhere from Schoharie county to Lake 

 Erie : among the principal localities may be mentioned the shores of Seneca and 

 Cayuga lakes; the shore of Canandaigua lake; Geneseo, Moscow, York, Covington, 

 Darien; and on the shore of Lake Eric at Hamburgh and Eighteen-mile creek. 



