80 
PALAEONTOLOGY OF NEW-YORK. 
growth give a more uneven surface, and this character becomes more prominent with in¬ 
creasing age. 
Although the general and prevailing form is oval or ovate, yet we not unfrequently meet 
with forms that are roundish, and the ventral valve wider than long. Increasing the variety of 
form and appearance, we may add, that the shells are often much broken, and appear to have 
been long macerated in water before becoming imbedded, so that perfect specimens are rare. 
In localities west of New-York, this fossil occurs mostly in the form of casts, and presents 
a still greater variety of form than in the localities in this State. At Springfield and Dayton, 
Ohio, it occurs in yellowish gray limestone, the prevailing forms being apparently the trilobate 
and gibbous. I am indebted to I. A. Lapham, Esq., of Milwaukie, for some very fine casts 
from that locality, exhibiting in the greatest extreme the large gibbous and depressed trilobate 
forms. 
I have collected this fossil, in the form of silicified casts, on the west side of the Mississippi 
river, having the same general characters as those from New-York localities; and I have a 
single specimen in translucent quartz, collected by the late Mr. Nicollet still farther to the 
northwest*. When to these localities we add its occurrence in Europe, we find this species one 
of the most persistent and widely distributed. So far as we know, also, it is the earliest created 
type of the genus; and though under its ordinary phases it has little of the form or aspect of 
the more recent species of the genus, nevertheless, in its various modifications it typifies many 
of those found in strata of succeeding groups. 
The following figures illustrate, to some extent, the principal varieties of form that have 
fallen under my observation. Those not otherwise indicated are from New-York. 
Plate XXV. 
Fig. 1 a. Ventral view of a perfect specimen, the form being somewhat depressed, and slightly 
indicating the trilobate character of the base. This is an extremely neat and beautiful 
specimen of the species. 
Fig. 1 b. The dorsal valve of a larger specimen, somewhat trilobate in form. 
Fig. 1 c. The dorsal valve of a smaller specimen, more distinctly trilobate. 
Fig. 1 d. The ventral valve less elongated and proportionally wider than the prevailing forms. 
Fig. 1 e. The dorsal valve of a small specimen, having the trilobate form, with an unusually 
thick and prominent beak. 
Fig. 1 /. A ventral valve nearly circular in form, being somewhat wider than long. 
Fig. 1 g. A small dorsal valve of nearly circular form, excepting the prominence of the beak. 
Fig. 1 A. A small ventral valve of circular form. 
* This specimen presents an interesting feature, not noticed in any other of this species in my collection, viz. a 
mesial depression in the ventral valve,, which produces aft extension or elevation in front, filling a sinus in the 
dorsal valve; this feature being the reverse of what it is observed in similar forms of Atryfa and other Brachiopoda, 
but characteristic of Pentamerus, as is distinctly seen in species of younger geological age. 
