136 CAKBONIFEIIOUS PERIOD. 



the full breadth of the shell in front of it ; lateral and front borders regu- 

 larly and continuously rounded. 



The dorsal valve would be almost chcular but for its titmcation by the 

 liinge-line, nearly flat, but slightly convex at the umbo, and sometimes 

 slightly concave at the front; beak minute, not prominent; area very nar- 

 row. 



Ventral vajve capacious, especially its posterior portion, which extends 

 much behind the hinge-line, and ends in a prominent, strongly incurving, 

 pointed beak ; area very narrow, high, concave ; mesial sinus absent, but 

 in its place there is usually a slight flattening at the front, and sometimes 

 an indistinctly impressed line is to be seen extending from beak to front. 



Surface apparently smooth, but under a lens it is seen to be finely 

 granular, the apparent granules being the bases of minute setae ; a few con- 

 centric lines of growth are usually observable upon both valves. 



This shell agrees so closely in many respects with S. Urii Fleming, 

 from the British Carboniferous strata, that the propriety of placing it under 

 any other specific name may well be questioned. In view, however, of the 

 fact that the characteristics of this subgenus admit of the development of 

 very few salient specific characters, I am at present disposed to regard 

 these minor differences as affording sufficient reason for continuing the use 

 of Shumard's name. It is one of the most common species in American 

 rocks of the Carboniferous period, ha^ang been found in the Coal-Measm-e 

 strata of Virginia, Pennsylvania, Illinois, Missouri, Iowa, Nebraska, and 

 Kansas, besides the regions that have supplied it to our collections, being 

 generally most abundant in the upper portion of the series. It is often gre- 

 garious ; some portions of the strata in which it occurs being composed 

 mainly of shells of this species alone. The collections, however, contain 

 comparatively few examples. 



Position and locality. — Strata of the Carboniferous period : near Santa 

 Fd, New Mexico ; and at Elko Mountain, Nevada. 



Spirifer (Martinia) glaber var. contracta Meek and Worthen. 



Plate X, fig. .2 a, J>, and c. 



Anomites glaher Martin, 1809, Petriflcata Derbiensia, pi. xlviii, figs. 9, 10. 

 Hpirifcr f/Iabcr Soweiby, 1820, Mineral Coiicbology, iii, 123. 



