164 PALEONTOLOGY OF NEW-YORK. 
Fig. 4 c. Profile view of the same. The specimen appears to be inequivalve, which is due to the sliding 
down of one valve below the other, as is clearly shown in the lower part of the figure, and 
in 4 8, 
Fig. 4 d. A portion of the surface enlarged, showing the fine undulating strie crossed by the concentric 
lines of growth. 
Position and locality. In the central part of the Trenton limestone at Middleville, Trenton 
Falls, &e. (Cabinet of Mr. Moore, of Trenton Falls.) 
213. 2. AMBONYCHIA ORBICULARIS. 
Pu. XXXVI. Figs. 5 a, b, ¢, d. 
Pterinea orbicularis. Emmons, Geol. Report, 1842, pag. 397, fig. 3. 
Rhomboidal or suborbicular, very ventricose in the middle and upper parts of the shell, 
regularly curved anteriorly, and somewhat compressed and alate behind ; umbones ven- 
tricose, narrowing above, very prominent, extended and incurved into a kind of hook or 
claw, which turns forward at the extremity ; cardinal line moderately extended, straight ; 
surface of the shell ornamented by fine radiating striae, which are crossed by sharp concentric 
elevated lines; cast smooth, marked by the large oval muscular impression of the shell. 
This species, in its young state, bears considerable resemblance to the preceding one ; 
but the radiating striz are always stronger, and distinctly marked by concentric lines ; and 
the shell is wider, though it does not attain so great a proportional width in the young as 
in the older specimens. It is not rare in the concretionary limestone at Watertown, where it 
is usually found in the condition of a cast, the material of the shell forming a thin crystal- 
line coating upon the surface, in which the original structure is not apparent. In a few 
specimens, I have detected small portions of the beautifully marked shell represented in 
fig.5 d. The proportions of height and breadth, shown in the figures, sometimes varies ; 
but the usual forms of perfect specimens are there represented. 
The associates of this shell are a single species of TELLInomya (T. anatiniforms) , 
Modiolopsis faba, Edmondia subangulata, and a slender Orthoceratite. The Edmondia 
subtruncata, E. ventricosa and E. undata, occur in a higher position, where the present 
species 18 never seen. 
Fig. 5 a. Right valve of a specimen, showing the muscular impression. 
Fig. 5 b. Left valve, retaining a portion of the shell, which is beautifully ornamented by radiating and 
concentric strie. 
Fig. 5c. Anterior profile view of a single valve. 
Fig. 5 d. A portion of the shell enlarged. 
Position and locality. This shell occurs in the lower concretionary and irregularly bedded 
Trenton limestone at Watertown, Jefferson county. 
