548 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



sprang as bifurcations from the two prominent bounding ribs. Tims there 

 are normally three plications in the sinus. In addition, the shell is marked 

 by about thirty- two radii, sixteen on either side of the sinus, which makes 

 an aggregate- of thirty-seven plications for the entire valve. Usually at 

 maturity these are all nearly or quite of a size. Sometimes, however, the 

 two bounding ribs remain prominent, while those in the sinus are almost 

 undeveloped. This constitutes the variety S. albapinensis, and represents 

 the condition of most young shells. Bifurcation is common amonp* the alar 

 ribs, but occasionally a shell will have nearly all its plications bifurcate. 



The area of the dorsal valve is narrow; beak small, inconspicuous. 

 There is a low but usually well-marked fold, on which at first are only two 

 plications. These subsequently bifurcate, giving four plications on the fold 

 of mature shells. Bifurcation is centrifugal and does not result in the 

 formation of primarily equal ribs. The narrower ridges are in each case 

 toward the wings, while the two original ribs remain central and contiguous. 



The surface ornamentation consists of fine radiating striae, parallel to 

 and superimposed on the radiating ribs. These are interrupted by a system 

 of concentric imbricating lamellae, as in S. biplkatus. 



The chief deviation from the type above described consists in an 

 increase in the number of strife, accompanied by a diminution in their size, 

 so that four or five stria? are found in the sinus, and twenty or more on the 

 wings. 



I believe that this species will be found to be synonymous with S. 

 biplkatus, but until proved I have employed the name of the better-known 

 form. 



Formation and locality: Madison limestone, near summit of ridge, west 

 end of Hunter Peak, Absaroka Range; limestone bluff north of Little 

 Sunlight Creek, Absaroka Range, 600 feet above the stream; east side of 

 Gallatin River, west of Electric Peak; Upper Gallatin Valley, west of Big- 

 horn Pass; Arnold Hague. Upper Gallatin Valley, divide between Galla- 

 tin River and Panther Creek, Gallatin Range; east face of Antler Peak, 

 Gallatin Range; saddle west of Antler Peak, Gallatin Range; amphitheater 

 west of Bannock Peak, Gallatin Range, bed 26; amphitheater east of Ban- 

 nock Peak, Gallatin Range, bed 30; W. H. Weed. Crowfoot Ridge, Gal- 

 latin Range, bed 25; J. P. Iddings and W. H. Weed. Same, top of bed 

 25; J. P. Iddings and G. M. Wright. Same, top of bed 26, lower part of 



