240 
PALEONTOLOGY OF NEW YORK. 
It is 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. oblongus 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. oblongus, 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 evei- strongly developed, and in any considerable collection of specimens it is easy to 
demonstrate the gradation of this species from P. pergibbosus. In association with this shell at Guelph 
there occurs an undescribed species of a well-defined Conchidium, not unlike the plicated shell referred 
by Whitfield (op. cit., p. 314, pi. xxiii, tigs. 1, 2,) to P. occidentalis. 
