544 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



on the fold, while the former regularly has five fine ones. L. haguei also 

 resembles to a certain extent L. JceUoggi of the Hamilton group, but it is 

 more gibbous, with a higher fold, while the latter has, as a rule, six to ten 

 plications on fold and sinus, with five or six faint lateral plications. 



Comparison can also be made with Liorhynchus (Pugnax?) striatocostatum 

 Meek and Worthen, which has three or four plications on the fold and two 

 or three nearly obsolete ones on either side. The fold is lower, broader, 

 and more quadrate, and the plications surmounting it are much coarser, 

 than in L. haguei, which has the lateral plications either entirely lacking 

 or more indistinct. The latter appears also to be without the fine striate 

 surface ornamentation of Meek and Worthen's species. 



Formation and locality : Madison limestone, Crowfoot Ridge, Gallatin 

 Range; cherty limestone, top of bed 24; A. C. Gill. 



DIELASMA King, 1859. 



Dielasma utah Hall and Whitfield. 



PL LXIX, figs. 6«, 66, 6c. 



Terebratula utah Hall and Whitfield, 1877: King's Geol. Expl. 10tli Par., Vol. IV, 

 p. 258, PI. IV, fig. 18. 



The type of this species is a dorsal valve, with a pentagonal outline 

 and a punctate shell. The rectilinear outline is not constant, however, for, 

 on a block of limestone from the same locality containing other type mate- 

 rial (Athyris planosulcata ?), is a second specimen whose outline is regularly 

 curved. 



The material from the Yellowstone National Park agrees with the above 

 so perfectly in general form, proportion, size, etc., that I am confident both 

 belong to the same species. 



Although it has not been possible to determine the internal charac- 

 ters of this form, it has been referred to the g-enus Dielasma because of its 

 geological position, punctate shell, and characteristic shape of the ventral 

 valve. 



Dielasma utah very closely resembles D. formosum Hall, which occurs 

 in the Mississippi Valley, in strata of Keokuk age; but D. utah is found 

 in the lower beds of the Madison limestones. 



