9 



the angularity, posterior to the end of the hinge line, and mer- 

 ges into the general convexity of the shell, toward the post-basal 

 extremity. Dorsal side of each valve flattened and inclined from 

 the post umbonal slope to the external ligament. Surface marked 

 by strong concentric lines of growth below the post umbonal slope 

 but between that and the hinge line the concentric lines are much 

 smaller. 



This is a marked species and no comparison with any other is 

 necessary. 



Found by R. A. Blair, in the Chouteau limestone, near Sedalia, 

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



SPHENOTUS SINUATUS, n. sp. 



Plate II, Fig. 29, right valve of large specimen; Fig. 30, left 

 valve of a smaller specimen believed to belong to 

 the same species. 



Specimens variable, in size, from one-half as large as the 

 smaller one illustrated to one-fourth larger than the larger 

 illustration. Shell trapezoidal. Cardinal line straight or slightly 

 arched. Anterior end gently rounded below the beaks. Posterior 

 margin obliquely truncate and then rounded into the basal margin. 

 Basal margin broadly and slightly confetricted in the middle part. 

 Valves depressed convex on the sides but more gibbous in the 

 umbonal region. Length twice as great as the height. Beaks at 

 the anterior end, small, flattened and appressed. Umbones angular 

 from the beak down the posterior slope to the post inferior ex- 

 tremity. Post cardinal slope convex and marked along the center 

 by a median angular ridge, in some specimeus, and by two 

 median ridges separated by a concave furrow in others, which 

 extend to the lower end of the oblique truncation of the posterior 

 margin. On our larger specimens, which we regard as the types 

 of the species, there are two median ridges, on the posterior 

 cardinal slope, and only one, on the smaller specimens. Cincture 

 oblique and extends from the beak to the constriction in the basal 

 margin. Surface marked by concentric lines of growth. All of 

 our specimens are casts. No part of the shell is preserved. 



This is a marked species and no comparison with any other is 

 necessary. 



Found by R. A. Blair, in the Chouteau limestone, near Sedalia, 

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



—2 



