108 PAL/EONTOLOGY OF ILLINOIS, 



ior side very short and regularly rounded; basal margin form- 

 ing a very gentle curve that is most prominent near the anterior 

 margin; posterior extremity contracted and narrowly rounded; 

 dorsal margin nearly straight, beaks 'depressed, incurved and 

 located about one-third the length of the shell from the anterior 

 margin. Surface markings unknown. 



Length of an internal cast in limestone 1% inches; height of 

 same % inch, convexity about % inch. 



This species may be readily distinguished from S. cliesterpnsis, 

 which it approaches in size and form, by its relative proportions 

 and more depressed beaks. 



Position and locality: The upper part of the Keokuk lime- 

 stone, Warsaw, 111. 



Collector, A. H. Worthen. 



No. 2500 of the Illinois State Museum. 



SCHIZODTJS NAUVOOENSIS. WortllPll. 



PL XX, Fig. 3. 



Schizodiis nauvooensis, Worthen, Match, 1884. Bulletin No. 2, of the Illinois State 

 Museum of Natural History, page 10. 



This shell is only known from a cast of a single valve in lime- 

 stone, but it differs so decidedly from all others of this genus, 

 that it may be readily characterized as follows: 



Shell above the medium size, obliquely sub-ovate, rather 

 strongly convex from the beak obliquely downward for about 

 half the length of the valve, and thence gradually depressed to 

 the ventral margin, anterior extremity regularly rounded from 

 the dorsal to the ventral margin, posterior side oblique, the 

 marginal line curving in below the beak; ventral margin not 

 entire, beak pointed, elevated and incurved, and situated near 

 the anterior margin, surface markings unknown. A well-defined 

 ovate muscular scar is conspicuous just below the beak, and 

 near the anterior border of the shell. 



Length from the beak to the ventral margin measuring obli- 

 quely, 2 inches; greatest breadth !% inches. 



Position and locality: Keokuk limestone, Nauvoo, 111. 



Collector, A. H. Worthen. 



No. 2499 of the Illinois State Museum. 



