570 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



beyond all question. Not so the specimens figured by Meek, 1 and doubt- 

 fully referred by him to LeVeillS's species. In comparing this form with 

 the one under discussion, I find it different not only from it but also from 

 the European type. The shell is large, thick, transversely elliptical; fold 

 but slightly defined, sinus broad and shallow, with a sharp median sulcus; 

 surface lamellose with thick layers, which, on the best-preserved surfaces, 

 show no trace of having been produced into spines. The ventral beak is 

 rather large, not strongly incurved, and instead of being furnished with a 

 round pedicle aperture, as shown in the figure, a careful study of the rostral 

 portion discloses that this appearance is due to a fracture which has also 

 partially removed the shell covering the arch of the beak, and that below the 

 point where the pedicle opening is indicated in the figure there is the anom- 

 alous character of an open triangular delthyrium. These facts are 

 developed from a study of the type material itself, which in the figured 

 specimen alone preserves the ventral beak entire, or nearly so. As the 

 form seems to be distinct from anything yet described from the same 

 horizon, I propose for it the name Athyris mira. 



The specific identity of CI. roissyi Walcott with LeVeiHe"^ species I 

 hold to be doubtful. 



That form as figured by its author is smaller (34.5 mm. in width by 22.5 

 mm. in length) and more deeply folded, the two depressions defining the 

 fold being so deep and triangular as to give the shell a trilobate appear- 

 ance. The fold is surmounted by a faint sulcus (!), resulting in a slight 

 emargination of the anterior border. Beak not incurved, so that the open 

 round foramen is a noticeable character. The surface is marked by not 

 very numerous but strong lamellar expansions, whose ragged edges suggest 

 that they may have been the origin of the characteristic spinose orna- 

 mentation of the genus Cliothyris. In the specimens from the Yellowstone 

 National Park and the Eureka district the fold and sinus are scarcely 

 discernible. The latter has a small, sharp beak, whose deep incurvature 

 completely conceals the foramen. That from the Yellowstone National 

 Park shows an indefinable aperture, partly beneath the beak, partly broken 

 through it. This contradiction is doubtless due to the same displacement 

 which makes the beak of one specimen seem so small, that of the other 

 comparatively so large. 



'1877. King's U. S. Geol. Expl., 40th Par., Vol. IV, PI. IX, figs. 3, 3a, 3b. 



