154 carboniferous period. 



Orber DYMYAEIA. 

 Tamily TRIGONID^. 



Genus SCHIZODUS King, 1844. 



Schizodus Wheeleri Swallow. 



Plate XI, fig. 6 a and b. 



Cypricardia ? Wheeleri Swallow, 1862, Trans. St. Louis Acad. Sci., ii, 96. 



Schizodus obscurus Geinitz, 1866, (Jarbonformat. und Dyas in Nebraska, 20 (not Sowerby, 



1821). 

 Schizodus Wheeleri Meek, 1872, U. S. Geol. Surv. Nebraska, 209. 



Shell of moderate size, irregularly subtrihedi'al or subovate in outline ; 

 posterior portion laterally compressed ; anterior portion inflated ; umbones 

 elevated ; beaks incurved, placed about one-quarter of the length of the 

 shell from the anterior extremity ; margins of the front and of the anterior 

 part of the base fonning a continuous and'regular curve ; basal margin slop- 

 ing upward, and meeting the downward and backward slope of the posterior 

 margin at a prominent angle, which is abiaiptly rounded ; dorsal margin 

 straight, sloping a little downward from the beaks to the obliquely-trun- 

 cated posterior margin ; posterior umbonal slope prominent, sometimes 

 forming a rather distinct ridge, which ends at the prominent angle of the 

 posterior margin and considerably increases its projection. Surface appar- 

 ently unmarked except by concentric lines of growth. 



Length, from front to posterior angle, thu-ty-one millimeters ; height, 

 from base to top of umbo, twenty-one millimeters. 



Our examples are all natural casts, the shell-substance having in all 

 cases been removed ; but the characteristics of the species are so distinctive 

 that it is recognized with little or no difficulty in that condition. It is a 

 common species in the Ujoper Coal-Measures of Missouri, Illinois, Iowa, and 

 Nebraska ; Professor Swallow's type-specimens of the species having been 

 obtained in the first-named State. 



Position and locality. — Strata of the Carboniferous period ; near Bear 

 Spring, Camp Wingate, New Mexico. 



