10 



ORDER ASIPHONIDA. 



Family ARCID^. 



MACnODON FACETU8, U sp. 



Plate I, Fi(j. 14, cardinal view; Fig. ]'>, right valve, both ends 



of the specimen are broken; Fig. IC, surface oj the 



cast magnified six diameters. 



Species small Our specimens are casts but part of the surface 

 markings of the shell are preserved. Shell elongate, about or 

 more than twice as long as high; highest about the anterior third. 

 Valves convex and thickness about equal to the height. Cardinal 

 line straight and a little short of the greatest length of the shell. 

 Anterior end angular at the cardinal line and then rounded into 

 the basal margin. Posterior end obliquely truncated, from the 

 end of the cardinal line, and then narrowly rounded, at the post- 

 basal extremity, into the basal margin. Basal margin broadly and 

 somewhat evenly rounded. Beaks near the anterior end and in- 

 curved above the hinge line. Umbones prominent and fading 

 into the general convexity of the shell. I'ost-umbonal sloiic sub- 

 angular and extending to the post-basal extremity. No cincture. 

 Cardinal slope concave. Surface marked by concentric lines of 

 growth that are plainly visible to the unaided eye, and by fine 

 radiating stri.e that give a beautiful cancellated structure to the 

 cast under an ordinary magnifier. This surface ornamentation is 

 doubtless plainly visible on the shell itself. 



This species will be readily distinguished by its general form, 

 obli(iuely truncated, posterior end, concave cardinal slope and 

 surface ornamentation. 



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

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



MACRODON I'ETTISENSIS, n. sp. 



Plate II, Fig. 17. right valve: Fig. IS, right ralve of a larger 

 specimen, icilh the (interior entt broken o(f. 



Species below medium size. SIkOI subelliptical or subovate, 

 wider behind. Length about one ami two-tliinls Ihe iieigiit. 

 Cardinal line straight and almost e(iuHling the greatest length of 

 the shell. Anterior end gently rounded into the basal margin. 

 Posterior end somewhat subtruncated in the upper part and 



