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PALAEONTOLOGY OP NEW-YORK. 
along its inner margins.’I have not observed any extensions of dental 
lamellse into the cavity of the shell. (There appear to be no proper dental 
plates, but simply a thickening of the margins of the fissure.) The 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 marks of this kind. 
The small shell in the Mar.cellus shale which I originally designated as Ortkis 
nucleus , 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 fine state of preservation. The illustrations of the interiors of the valves are 
of specimens from this group. 
2 3 4 -5 6 
Figs. 1, 2, 3. Dorsal, ventral and profile views of Ambocce.Ua umbonata. natural size, from the shales of 
the Hamilton group. 
Fig. 4. The interior of the ventral valve showing foramen, area, etc. enlarged. 
Fig. 5. Interior of the dorsal valve enlarged, showing the foveal plates, dental sockets, and the quadri¬ 
partite muscular impression; the valve slightly distorted. 
Fig. 6. A similarly enlarged dorsal valve, showing some variations from the preceding. 
Geological formations and localities. In the Marcellus 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 Erie at Hamburgh and Eighteen-mile creek. 
