240 PALEONTOLOGY OF NEW YORK. 



It 18 evident from the foregoing remarks that Pentamerus is an exceedingly 

 plastic type, and its duration is essentially the same in all countries. Davidson 

 finds P. oblongtts restricted to the lower and upper Llandovery rocks. In the 

 Baltic Provinces of Russia the species P. borealis, von Eichwald, a shell very 

 similar to that variety of P. obhngus, here termed subrectus, occurs in agglom- 

 erations (Borealis-bank) like those of P. oblongus in the Clinton limestone, at a 

 low horizon in the upper Silurian. Above this is the zone of P. Esthonus, 

 another form very close to P. oblongus. In Scandinavia, P. oblongus is found in 

 a higher horizon, which corresponds more nearly with its occurrence in the 

 Niagara dolomites of the interior States. P. Samojedicus, Keyserling, from 

 the Petschora-land is still another smooth species from a corresponding 

 horizon. 



shell subquadrate in transverse section. The surface, in the usual preservation of the shell, has strong con- 

 centric, often squamous growth-lines and distinct traces rf radiating plications. It is doubtful if these 

 plications were ever strongly developed, and in any considerable collection of specimens it is easy to 

 demonstrate the gradation of this species from P. pergibhosus. In association with this shell at Guelph 

 there occurs an undescribed speines of a well-<letined Conchididm, not unlike the plicated shell referred 

 by WuiTPiKLD {op. cit., p. 314, pi. xxiii, figs. 1, 2,) to P. occidentalis. 



