296 rAL.TEONTOLOGY. 



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

 ])o.sterior end broadly truncate and slightly rounded. Body of the valves 

 very ventricose, subangular along the umbonal ridge, mai'ked by strong 

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

 eight of these costae 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 squarely-depressed grooves, corresponding to the external 

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



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

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



esq. 



Septocaudia Cakditoidea u. sp. 



riato VII, fig. 25. 



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

 row anteriorly, and widening behind. Valves ventncose, 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 narroAvly 

 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 

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

 mined; those on the postero-cardinal .slo])e near the basal angle count- 

 ing about live 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. Tlie cost;c are crossed transversely by fine, closely-arranged, 

 zigzag lines, strongly arched upward in i-rossing the ril)s. Greatest length 

 of the specimen, l;] inches; height, 1^ inches. 



Thi.s species difiers from S. tijpica not only in its greater size, but in 



