DEVONIAN FOSSILS. 505 



The shell is so badly exfoliated that its surface characters are unknown, 

 and the generic reference, as a whole, is doubtful. It resembles Athyris 

 vittata of the Corniferous and Hamilton groups, and A. angelica of the 

 Hamilton, some examples of which approach this shell, though not very 

 closely. It finds a close ally in A. angelica var. occidentalis Whiteaves 

 (Cont. Canadian Pal., Vol. I, Pt. Ill, 1891, p 227, PI. XXXII, figs. 3-Sa), 

 from which it differs chiefly in having an additional low fold on either side. 

 The two forms resemble each other more than either A. angelica or A. 

 vittata. Whiteaves's shell is probably worthy of specific distinction, and the 

 relation between it and the form from the Madison limestone would be best 

 expressed by making the latter a variety of the former. The name of my 

 shell would then be Athyris occidentalis var. triplicata. 



Formation and locality : Three Forks limestone, south side of Soda 

 Butte Creek, northwest of Abiathar Peak, Absaroka Range; J. P. Iddings. 



GASTROPODA. 



PLEUROTOMARIA Defrance, 1824. 



Pleurotomaria isaacsi Hall and Whitfield. (!) 



PI. LXVI, figs. 5a, 5b. 



Pleurotomaria isaacsii Hall and Whitfield, 1873: Tweuty-third Kept. New York State 

 Cab. Nat. Hist., p. 238, PI. XII, figs. 6, 7. 



The form which I have referred to Hall and Whitfield's species occurs 

 as an isolated specimen of a large gastropod shell. As far as the somewhat 

 imperfect condition of the material permits a comparison, the specimen from 

 Yellowstone National Park is very close to P. isaacsi. It is a large flattened 

 shell, about 59 mm. in diameter, agreeing in size, general proportion, and 

 peritreme section with the species to which I have referred it. On the 

 other hand, the spire is a little more elevated than the specimen figured by 

 Hall and Whitfield, the whorls a little more angular in section, with the 

 upper surface obliquely plane or slightly concave. The shell appears to be 

 without ornamentation. 



Pleurotomaria isaacsi is from the Lower Devonian, probably the Scho- 

 harie grit. 



Formation and locality: Three Forks limestone, Wall Canyon, Clark 

 Fork Valley; Arnold Hague. Lower Devonian, near Raymond Station, 

 Iowa. 



