100 PALBONTOLOGY 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 laminae ; but in the single specimen we have, their 

 edges are much broken. In some of its characters it approaches the O. 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, n a,b, c, d. 

 Orbinila terminalu. Conkad 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 subcorneal, 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 Ihe 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, pi. xix.), which possesses the same 

 remarkable feature of having the convex valve perfdrated 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 sanditone a micaceous appearance, 



