100 
PALAEONTOLOGY OF NEW-YORK. 
This shell is nearly flat, with the exception of the small apex. The concentric lines 
appear to be made up of thin shelly laminre ; but in the single specimen we have, their 
edges are much broken. In some of its characters it approaches the 0. rugata (Silurian 
Researches , pag. 610, pi. 5, fig. 11), a species, or an analogue of which, we find in great 
abundance in the Hamilton group ; the latter differs, however, in the concentric ridges 
being proportionally finer and less elevated, and the shell is much larger. The one under 
consideration is readily distinguished from any other in the Trenton limestone, by its slight 
elevation and subcentral apex. 
Position and locality. Middleville, in the shelly layers, lower part of the Trenton lime¬ 
stone. A rare species. 
140. 4. ORBICULA TERMINALIS. 
Pl. XXX. Figs. 11 a, b, c, d. 
Orbicula terminalis. Conrad in MS. Emmons, Geol. Report, pag. 395, fig. 4. 
Shell very obtusely subovate or orbicular ; ventral valve depressed convex, with a 
terminal or marginal apex ; dorsal valve very convex or subconical, with a central apex, 
a broad depression and narrow slit on one side, extending to the margin of the shell opposite 
the beak of the ventral valve ; surface usually smooth, from exfoliation? Fine longitudinal 
or radiating striae are also visible when the shell is exfoliated ; these striae are sometimes 
punctate. 
A single specimen, apparently in a perfect condition, is marked by strong concentric 
elevated lamellae, giving a rough aspect to the surface. 
This fossil is easily recognized by its low convex suborbicular ventral valve, with a 
marginal apex. The opposite valve is less often seen, but it is as readily distinguished by 
the broad depression on one side of the apex, with the narrow slit in the bottom for the 
protrusion of the peduncle. This character is a very remarkable one, and serves to dis¬ 
tinguish this from all other known species of Orbicula in our strata. 
M. de Verneuil, in his Palaeontology of Russia and the Ural Mountains , has described 
and figured a species of Orbicula (the O. reversa, pl. xix.), which possesses the same 
remarkable feature of having the convex valve perforated for the protrusion of the ligament. 
This rare character is possessed by few species of the Orbicula ; the Orbicula lodensis of 
the Geological Reports of the Third and Fourth Districts of New-York, being the only one 
previously known to me; and in this one, the perforated valve is far less convex than in 
the one under consideration. 
The Russian species occurs in an ancient Silurian sandstone in the vicinity of St. Peters- 
burgh, which is perhaps of the same age as our Potsdam sandstone. Associated with it is 
another species of Orbicula, and an Obolus, a genus allied to Lingula, and which in this 
instance occurs in great numbers, giving the Russian sandstone a micaceous appearance, 
