MESOZOIC FOSSILS. 631 



in diameter, from the divide between Fawn Creek and Gallatin Valley. 

 This is a nearly smooth, discoid, involute form, with narrow umbilicus and 

 rounded abdomen. The outer two-thirds of the body whorl appears to 

 have been entirely smooth. On the other third the abdomen is crossed by 

 small ribs, giving it almost a dentate outline; and on earlier stages these ribs 

 are relatively longer and more prominent, passing nearly halfway across 

 the flanks of the shell. In general form and sculpture this species resembles 

 Oppelia subplicatella Vacek, 1 from the Oolitic of Cap San Vigilio. The 

 septa are not preserved. 



Fragments of larger individuals that may belong to the same species 

 were obtained on Fan Creek Pass, saddle in ridge Avest of south head of 

 Gardiner, and from Cinnabar Mountain. They are less compressed than 

 the specimens above described, but with the material at hand it is impos- 

 sible to determine whether this difference is clue to accidental distortion. 

 Some of the fragments show strong plications on the abdomen. A large 

 specimen, 8 inches in diameter, from limestone on ridge south of Sheridan 

 Peak, appears to be related to the forms above mentioned, though it is 

 somewhat more involute, and is so much weathered that all the surface 

 characters and the finer subdivisions of the septa have disappeared. The 

 specimen is septate throughout, The septa appear not to have been very 

 complex and the lateral saddles are very broad. It is possible that this 

 specimen should be referred to Ammonites lienryi M. and H., which it some- 

 what resembles both in general form and in the septa. 



Perisphinctes sp. 



Collections . obtained by Dr. Peale near the lower canyon of the 

 Yellowstone contain fragments of two species of Ammonites that probably 

 belong to Perisphinctes, judging from the sculpture. Fragments of one of 

 these species were also obtained on saddle in ridge west of south head of 

 Gardiner River. 



Belemnites densus Meek and Hayden. 



Belemnites densus Meek and Haydeu, 185S: Proc. Acad. Nat. Sei. Phila., p. 58. 1865: 



Paheont. Upper Missouri, p. 126, PL IV, figs. lOa-c; PI. V, figs. la-i. Meek, 



1876 : Simpson's Kept. Expl. Great Basin, Utah, p. 358, PI. Ill, figs, ia, b. Whit- 

 field, 1880 : Geol. Black Hills Dakota, p. 3S1, PI. VI, figs. 15-19. 



This species, which is abundant in the Jurassic of the Black Hills and 



1 AbUandl. K.-k. geol. Reichsanst.ilt, Vol. XII, p. 82, PI. XI, figs. 1-5, 1886. 



