448 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. . 



A study of the material collected from the same horizon at a point not 

 far distant from the original locality shows the presence of a false area and 

 a pseudo-deltidium of the same character as that of Iphidea bella, the type 

 of the genus Iphidea. There is, however, a difference in the two forms that 

 might possibly be considered of subgeneric value. The surface of Iphidea 

 bella is covered with fine concentric stria?, while the surface of Iphidea 

 sculptilis is marked by very fine, sharp, elevated, concentric lines that 

 coalesce or bifurcate irregularly, imparting a peculiarly interrupted wavy 

 appearance that is highly characteristic. The variation in the surface 

 character is continued still further in Iphidea pannula White, in which the 

 surface is marked by a very fine network of oblique raised lines that divide 

 it into minute porelike pits, which cause it to resemble under the lens 

 the texture of finely woven cloth. The difference in character of surface 

 between Iphidea pannula and /. sculptilis is the same difference as between 

 the surface characters of Trematis and Lingulella; and the difference in 

 surface characters between the two species mentioned, on the one hand, 

 and Iphidea bella and I. labradorica, on the other hand, is equally important, 

 as the latter have the plain concentric striae and lines of growth that are 

 characteristic of the species, while the former have highly ornamented sur- 

 faces. If we should find the genus on the surface characters of the shell, it 

 would be necessary to place I. bella, I. labradorica, and I. prospectensis under 

 one genus, I. ornatella (Linnarsson) and I. scidptilis under another, and 

 I. pannula and I. ccelata under still another. There are, however, specimens 

 of /. pannula on which the outer portions of the shell show the surface 

 characters of I. bella; and more or less complete transitions may be found 

 between the varying types of surface ornamentation. 



There are too many similarities in form to warrant us in removing 

 I. sculptilis from Lphidea without the evidence of interior markings to prove 

 that it is generically distinct. 



Formation and locality: Middle Cambrian, Flathead terrane (lowest 

 fossiliferous bed); Crowfoot section, Gallatin Range, Yellowstone National 

 Park. It also occurs in the Middle Cambrian shales of Antelope Springs, 

 Utah. 



