630 GEOLOGY OF THE YELLOWSTONE NATIONAL PAKE. 



M. and H., which is much larger and more slender in form, differing in all 



its details from this species, which is somewhat similar in form and size to 



Neritina pisum Meek from the Upper Cretaceous of Utah. 



The type was collected by Prof. A. C. Gill about 3 miles southeast of 



Gravel Peak. 



Lysoma powelli White. 



Neritina poivelli White, 1876: Geol. Uinta Mountains, p. 110. 



Lyosoma powelli White, 1880: Ann. Rept. U. S. Geol. Surv. Terr, for 1878, p. 153, PI. 

 XXXVIII, figs. 6a-d. 



One well-preserved specimen was obtained on saddle at head of Fawn 

 Creek, northeast of Monument Peak, and another on ridge south of Mammoth 

 Hot Springs, on main terrace. The species has not before been reported 

 from any locality excepting at the mouth of Thistle Creek, Spanish Fork 

 Canyon, Utah. 



Both Zittel and Fischer are inclined to make Lyosoma a synonym of 

 Otostoma d'Archiac, an Upper Cretaceous subgenus of Nerita, but Lyosoma 

 really has the thin inner lip without any callus or flattening of the columella, 

 while the Cretaceous form has been shown to have the characteristics of 

 Neritina in these respects. 



TUERITELLA Sp. 



A single small specimen from 3 miles southeast of Gravel Peak has 

 the form of this genus, but is insufficient for specific description. It con- 

 sists of six flattened whorls with channeled sutures. 



Natica! sp. 



A naticoid form is represented by imperfect internal casts from Fan 

 Creek Pass, saddle west of south head of Gardiner, head of Fawn Creek 

 northeast of Monument Peak, and near Sentinel Butte. 



It is probably undescribed, being very much larger than Natica 1 lelia 

 Hall and Whitfield, which is the only described naticoid shell from the 

 Jurassic of this western interior region. 



CEPHALOPODA. 



Oppelia? sp. 



Ammonites are rare in the Yellowstone National Park collection, and 

 the few that Avere obtained are too fragmentary and badly preserved for 

 accurate classification. 



One species is represented hj, two flattened specimens, about 3 inches 



