296 PAL^Ol^TOLOGY. 



orly; anterior end short and obtusely pointed ; basal line gently rounded ; 

 posterior end broadly truncate and slightly rounded. Body of the valves 

 very ventricose, subangular along the umbonal ridge, marked by strong 

 radiating costse, separated by equally wide, flattened interspaces ; six or 

 eight of these costse occupy the postero-cardinal slope, and from twelve to 

 sixteen may be counted on the body and anterior portion of the shell. The 

 costse are flattened on the summit, and marked by closely-arranged, recurved, 

 transverse ridges, or nodes, which become more crowded and stronger with 

 the increased age of the shell. Interior margin of the valves strongly den- 

 ticulate from the squarelj^-depressed grooves, corresponding to the external 

 ribs, and which extend to nearly one-third of the width of the shell. 



Formation and locality. — In limestones referred to the Jurassic, at Sho- 

 shone Springs, Augusta Mountains, Nevada. Collected by S. F. Emmons, 



esq. 



Septocardia Caeditoidea n. sp. 



Plate VII, fig. 25. 



Shell of medium size, subrhomboidal in outline, longer than high, nar- 

 row anteriorly, and widening behind. Valves ventricose, very angular 

 along the umbonal bridge, rapidly sloping to the postero-cardinal margin, 

 and more gently toward the anterior end of the shell ; beaks large, promi- 

 nent, and strong ; placed well toward the anterior end, which is narrowly 

 rounded ; basal line broadly rounded ; posterior extremity obliquely trun- 

 cate, longest at the postero-basal angle. Internal features unknown. Sur- 

 face marked by numerous, strong, sharply-elevated, angular, radiating 

 costse, with sharply V-shaped interspaces, the exact number not deter- 

 mined ; those on the postero-cardinal slope near the basal angle count- 

 ing about five in the space of half an inch, giving about a tenth of an inch 

 from crest to crest ; toward the cardinal line they become gradually finer. 

 Those on the body of the shell appear to have been of nearly equal strength 

 with those on the posterior part, becoming finer toward the anterior 

 extremity. The costse are crossed transversely by fine, closely-arranged, 

 zigzag lines, strongly arched upward in crossing the ribs. Greatest length 

 of the specimen, 1| inches ; height, 1^ inches. 



This species differs from S. typica not only in its greater size, but in 



