HO 



PALEOBIOLOGY 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 h. 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 unusuall}'' 

 thick and prominent beak. 



Fig. 1 /. A ventral valve nearly circular in form, being somewhat wider than long. 



Fig. 1 ^. A small dorsal valve of nearly circular form, excepting the prominence of the beak. 



Fip-. 1 A. A small ventral valve of circular form. 



o 



*This specimen presents an interesting feature, not noticed in any otlier of this species in my collection, viz. a 

 mesial depression in the ventral valve, which produces an 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 Atrypa and other Brachiopoda, 

 but characteristic of Pentamerue, as is distinctly seen in species of younger geological age. 



