11 



rouuiled into the basal nmrj^iu. Tlic Imsal margin iu the subellip- 

 tical speciuK'US i8 broadly rouudcd, but, in tlie subovate speci- 

 niens, 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. Umboues gently convex with a slightly 

 angular posterior slope that merges into the general convexity of 

 the shell near the postero-biisal margin. Shell marked by regu'ar 

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

 the posterior unji)oual slope. 



This species most reseinblps lifticrodun humiUnnid', Init is dis- 

 tinguished from that species by being shorter in proportion to its 

 length, subangular posterior umbouiil slope, and the absence of 

 radiating lines, escei)t obscure ones on the posterior umbonal 

 slope. The surface ornamentation is a ])rominent feature in 

 M. hamiUoniir while the lines on this species are (juite incon- 

 spicuous, though our shells are finely preserved. 



Found by II A. Blair, iu the Chouteau limestone, near Sedalin, 

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



MACRODOX BLAIRI, n. Sp. 



Flaic II, Fig. 10, 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 slicll at the poslerior end : the 

 basal margin is eroded: Fig. 2i, 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 

 abru{)tly 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 iu the middle part. Beaks 

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



