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 b. 



Fig. 4 d. A portion of the surface enlarged, showing the fine undulating strice crossed by the concentric 

 lines of growth. 



Position and locality. In the central part of the Trenton limestone at Middleville, Trenton 



Falls, &C. (Cabinet of Mr. Moore, of Trenton Falls.) 



213. 2. AMBONYCHIA ORBICULARIS, 



Pl. XXXVI. Figs. 5 a, b, c, d. 

 Pterinea orbicularia. Emmons, Geol. Report, 1842, pag. 397, fig. 3. 



Rhomboidal or siiborbicular, 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 extreinity ; cardinal line moderately extended, straight ; 

 surface of the shell ornamented by fine radiating stria', 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 striae 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 heiglit 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 fnba, Edmondia suhangulata, and a slender Orthoceratite. The Edmnndia 

 subtrnncata, E. ventricosa and E. vndata, occur in a higher position, where the present 

 species is never seen. 



Fi.j. 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 stria;. 

 Fig. 5 c. 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. 



