INVERTEBRATES. 301 



lines of growth, in consequence of the sinuate outline of the posterior margin. 

 The compression of the beaks, mentioned above, is not in the direction of the 

 plane of the valves, but from the anterior and posterior sides, in consequence 

 of which their transverse diameter is considerably greater than that at right 

 angles to the same. 



Locality and position : Chester, Illinois; Chester group of Lower Carbon- 

 iferous series. 



Genus SCHIZODUS, King, 1844. 



(Ann. and Mag. Nat. Hist., p- 64.) 



Schizodus chesterensis, M. and W. 



PI. 23, fig. 6a, 6 b. 



Schizodus chesterensis, Meek and Worthen, Oct., 1865. Proceedings Acad. Nat. Sci., 

 Philad., p. 457. 



Shell rather above medium size, transversely ovate, convex 

 in the anterior and umbonal regions, cuneate posteriorly ; an- 

 terior side regularly rounded; base forming a broad semiovate 

 curve, being more prominent in the central and anterior 

 regions than behind ; posterior side contracted, longer than the 

 other, and narrowly rounded at the extremity; dorsal margin 

 nearly straight, and declining slightly behind the beaks, from 

 which it rounds regularly into the anterior border ; beaks gib- 

 bous, rather elevated, incurved, and located less than one-third 

 the length of the shell from the anterior margin. Muscular 

 impressions shallow; anterior ones ovate, placed near and 

 a little above the middle of the anterior margin; posterior 

 impressions ovate, located close up under the dorsal margin, 

 between the middle and the anal extremity. Surface marked 

 by fine, regular concentric striae. Length, of an internal cast, 

 1.38 inches; height of same, 1 inch; convexity, 0.68 inch. 



This species resembles Schizodus obscurus, of Sowerby, from the Permian 

 rocks, but is smaller, has less elevated beaks, and differs in having the most 



