11 



rounded into the basal margin. The basal margin in the subellip- 

 tical specimens is broadly rounded, but, in the subovate speci- 

 mens, the postero-basal margin is somewhat produced and more 

 abruptly rounded. Valves compressed or only moderately convex. 

 Beaks a little anterior to the middle, obtuse and rising slightly 

 above the cardinal line. Umbones gently convex with a slightly 

 angular posterior slope that merges into the general convexity of 

 the shell near the postero-basal margin. Shell marked by regular 

 concentric lines of growth, and a few obscure radiating lines on 

 the posterior umbonal slope. 



This species most resembles Macrodon hamiUonice, but is dis- 

 tinguished from that species by being shorter in proportion to its 

 length, subangular posterior umbonal slope, and the absence of 

 radiating liues, except obscure ones on the posterior umbonal 

 slope. The surface ornamentation is a prominent feature in 

 M. hamiUonice while the lines on this species are quite incon- 

 spicuous, though our shells are finely preserved. 



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

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



MACRODON BLAIEI, n. Sp. 



Plate II, Fig. 19, right valve of a large specimen; Fig. 20, left 

 valve; Fig. 21, left valve of a small specimen; Fig. 22, 

 cardinal view; Fig. 23, left valve of same show- 

 ing some of the shell at the posterior end; the 

 basal margin is eroded; Fig. 24, left 

 valve of a medium specimen. 



Specimens in this species variable in size, from small to large. 

 Shell elongated, about twice as long as high. Highest near the 

 anterior third. Valves convex, thickness about two-thirds the 

 height. Cardinal line straight and constituting the greatest 

 length of the shell. Anterior end angular at the cardinal line 

 and gently recurved to the basal margin like the prow of a boat. 

 Posterior end truncated to near the post-basal margin and then 

 abruptly rounded into the base; sometimes the cardinal line ter- 

 minates in an acute point and the truncated end is incurved to 

 near the post-basal margin. Basal margin broadly rounded or 

 nearly parallel with the cardinal line in the middle part. Beaks 

 near the anterior third of the shell, somewhat acute and rising 



