566 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



Hague. West of Antler Peak, Gallatin Range; A. C. Grill. Summit of 

 peak west of same; cherty belt, Bighorn Pass, Gallatin Range; Crowfoot 

 Ridge, Gallatin Range, top of bed 24; J. P. Iddings. Head of Conant 

 Creek, Teton Range; W. H. Weed. 



Seminula immatura n. sp. 

 PI. LXXI, figs. 5a, 5b, 5c, 5d. 



Shell rather small, ovate, gibbous. Length somewhat exceeding the 

 width. Dorsal valve full, highest at the umbo, without a fold, but slightly 

 emarginated in front; beak small, inflated, deeply incurved. Ventral valve 

 oval, beak small, incurved so as to conceal the true foramen. There is a 

 shallow sinus, which, however, is perceptible only near the anterior margin, 

 forming there an upturned projecting lip to fill in the emargination of the 

 opposite valve, and producing a sinuousity in their line of union. 



Length, 18.5 mm.; width, 15 mm.; thickness, 12.5 mm. 



Stratigraphically this is the lowest of the Seminulas obtained in the 

 Yellowstone National Park, of which S. madisonensis is the highest. The 

 obsolete fold and sinus, the regular, deeply arched valves, the evenly 

 rounded, ovate shape, the tumid dorsal umbo and incurved beak, and the 

 small resupinate ventral beak are all characteristic, and sharply differ- 

 entiate it from the latter. 



Compared with S. humilis, it is considerably larger, more tumid ; ventral 

 beak smaller and narrower; beak of the dorsal valve larger, more inflated 

 and incurved. 



Formation and locality : Madison limestone, west of Antler Peak, Gal- 

 latin Range; A. C. Gill. 



CLIOTHYR1S King, 1850. 



This genus (or subgenus) is practical^ coextensive with the species 

 CI. roissyi LeVeille", as the latter at present stands. Partly because the 

 character of the surface ornamentation renders it difficult to secure well- 

 preserved specimens, the discrimination of species in this group, if indeed 

 it has been systematically attempted, has not, I believe, proved successful. 



The Athyris roissyi question has thus become too complicated through 

 the prolonged sedimentation of cliothyroid forms to admit of its ready 

 solution. That it is desirable to subdivide this group is obvious ; that it will 



