136 



[ Senate 



implantation and bifurcation, crossed by very fine indistinct 

 concentric lines and occasional more distinct concentric imbri- 

 cating marks of groAvtli- entire surface granulate or punctate 

 under a magnifier. 



This species is so nearly like O. micMlina of Levetlle, that it is very 

 difficult to point out characters by which they may be distinguished, espe- 

 cially until that species is better defined. From the figures of that species, 

 ours presents considerable differences in the vascular impression and dental 

 processes. The minute granulations or punctse upon the exterior surface 

 present characters whicb are not noticed in the descriptions and figures, so 

 far as I have observed. 



The minutely tubular strire are characteristic of so many species of Or- 

 ihis, that it appears to me this character can be of no value in distinguishing 

 species, unless its peculiarities are pointed out. A single European specimen 

 of Orthis michilina, from the Carboniferous rocks of Belgium, in my 

 cabinet, presents a more elongated and less gibbous form, a flattening along 

 the centre of the dorsal valve, and a proportionally more extended area. 



Geological position and locality. In the shales and shaly sandstones of 

 the Hamilton group in Eastern and Central New- York, and more abun- 

 dantly in the western part of the State. It likewise occurs in the soft sand- 

 stones of the age of the Chemung group in Eastern Ohio. Some small and 

 poorly developed specimens of Orthis in the shales of the Hamilton group 

 in Iowa closely resemble this species ; but the rotund and gibbous forms 

 never appear there, and the identity of the two is not determined. 



OaiHIS VANUXEHI. 



