17 



Family CYTHERODONTID^. 



SCHIZODUS SEDALIENSIS, D. Sp. 



Plate II, Fig. 15, cardinal view; Fig. 16, right valve. 



Species medium or below medium size. Shell subquadrate. 

 Our specimens are casts, and somewhat compressed or only mod- 

 erately convex. Length somewhat greater than height. Anterior 

 end rounded. Basal margin broadly rounded. Posterior end sub- 

 truncate and gradually rounded into the basal margin. Cardinal 

 line straight and prolonged posteriorly which produces the sub- 

 quadrate outline to the shell. Beaks anterior to the middle of the 

 shell, acute, and rising above the cardinal line where they are 

 slightly incurved. Posterior umbonal slope somewhat angular and 

 fading away toward the lower posterior part of the shell. Ante- 

 rior umbonal slope rapidly merges into the depressed convexity or 

 cuneiform shape of the shell. Pallial line regularly curves from 

 one muscular scar to the other. Posterior end of the cardinal 

 line subalate. Surface of the casts smooth and outline of the 

 muscular scars not indicated. 



This species is readily distinguished from all others, but is 

 probably as nearly related to S. medinensis as to any other. But 

 the shell of that species has a subtrigonal outline, while the out- 

 line of this species is subquadrate. 



Found by II. A. Blair, in the Chouteau limestone, at Sedalia, 

 Missouri, and now in the collection of S. A. Miller. 



Remarks — We have found no species among the Lamellibranchs, 

 from the Chouteau Group of Missouri, that occurs in any higher 

 or lower Group of rocks. The genera, however, with the exception 

 of Blairella, the new genus above described, are known to range 

 geologically, as follows: Posidonomya, from the Upper Silurian 

 to the Coal Measures; Macrodon, Schizodus, Lunidicardium and 

 Edmondia from the Devonian to the Coal Measures; Elymella and 

 Sphenotus from the Devonian to the Chouteau; Cypricardella from 

 the Devonian to the Warsaew; Myiilarca from the Upper Silurian 

 to the Chouteau. Lioptcria and Palwosolen were, heretofore, 

 known only from Devonian rocks, and their range is now extended 

 to the Cnouteau. Chmnomya was, heretofore, known only from 

 the St. Louis Group to the Coal Measures and it is now brought 

 down to the Chouteau. Species, heretofore described, from the 



