ORISKANY PERIOD. 



267 



2. Ai 



ds. 



The most common species are the coarse Spirifer arenosus (fig. 442), 

 and the JRensselaeria ovoides (fig. 444). The rock is often made up 



Figs. 442-444. 



Brachiopods. — Figs. 442, 443, Spirifer arenosus; 444, Rensselaeria ovoides. 



of these large fossil shells crowded together, or contains their casts 

 with the cavities the shells once occupied. Fig. 443 represents a 

 cast of the interior of Spirifer arenosus. 



In New York, the fossils include Brachiopods (which are the most numerous 

 species), Conchifers, Gasteropods, Ceplialopods (Orthocerata, etc.), and Trilobites, 

 and only traces of Crinoids. In Maryland, according to Hall, there are a number 

 of fine Crinoids of the genera Mariacrimis, Edriocrinus, and others, besides three 

 species of Cystideans, and among them one of the peculiar genus Anomalocystis 

 (allied to fig. 418). Among the Gasteropods of the same region there are great 

 numbers of shells of the genus Platyceras, — a thin conical shell having the top 

 rolled to one side (like fig. 458), related apparently to Janthina of our seas. In 

 some places in Maryland and Virginia, they occur packed crowdedly together in 

 a soft sand-rock, the sands of which are hardly more coherent than those of a 

 sea-beach (Hall). This rock contains a wonderful profusion of shells, although 

 the number of species is small. The ribs of some of the Spirifcrs have a pecu- 

 liarity observed in only one American Silurian species (of the Niagara epoch), 

 but in Europe not known before the Devonian age, — which is, that they subdivide 

 dichotomously, instead of being simple. 



The shell in the genus Rensselaeria Hall, contains a loop-like arm-support, a 

 little like that in Terebratula, but it is only curved, instead of bent, and has a 

 spade-shape termination. 



