564 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



This shell occurs in the upper beds of the Madison limestone, and as 

 it bears at first sight a strong resemblance to the general type of S. subtilita 

 Hall, I referred it to that form. A comparison with specimens of 8. subtilita 

 from the type locality, near Weston on the Missouri River, reveals certain 

 differences which seem to prove the two forms distinct. 



S. subtilita 1 is somewhat larger than 8. madisonensis, narrower in pro- 

 portion to its length, with the widest portion near the anterior margin, so 

 that the outline is subtriangular. S. madisonensis, on the other hand, is 

 widest near the middle of the shell, and the outline is pentagonal. 



The sinus in S. subtilita is broad and shallow, not apparent more than 

 one-third the shell length back of the anterior margin. The anterior sinu- 

 osity is nearly rectilinear, with subparallel sides. The dorsal valve, on the 

 contrary, can scarcely be said to have any corresponding fold, as that 

 structure falls into the general curvature of the valve, which is highly 

 arched. In 8. madisonensis the sinus is no deeper than in 8. subtilita, but it 

 is angular and can be traced back to the umbonal region. The sinuosity 

 is triangular and the fold is defined for a short distance by lateral sulci. 



In shells of the same size S. subtilita is more obese than the other, the 

 dorsal valve especially being highly tumid, particularly about the beak, 

 which is narrow, high, and pinched. The ventral beak also is more deeply 

 incurved than is the case in 8. madisonensis, and the rostral angle is more 

 acute. 



Formation and locality: Madison limestone, head of Conant Creek, 

 Teton Range; W. H. Weed. 



Seminula madisonensis, var. pusilla, n. var. 

 PI. LXXI, figs, da, 3b. 



In bed 28, Crowfoot Ridge, Gallatin Range, and also probably 600 

 feet above the stream at the limestone bluff north of Little Sunlight Creek, 

 Absaroka Range, occurs scantily a small Seminula which I provisionally 

 refer to S. madisonensis as a variety of the same. It is specifically distinct 

 from other members of the genus observed, and is perhaps distinct from 

 8. madisonensis also. 



I have seen only four or five specimens of this type, but these indicate 

 that it is much smaller than S. madisonensis, and more elongate; the ventral 



1 See PL LXXI, figs, ia-id. 



