578 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



heads and larger pygidia were referred by the author to P. peroccidens, and 

 the small type of pygidium was described as P. loganensis. 



In the Yellowstone National Park collections no pygidia of the type 

 of P. peroccidens (the unornarnented form) have been observed, but the 

 large heads have been identified as P. peroccidens, and the small pygidia as 

 P. loganensis. As the pygidium and a large head occur associated at the 

 same locality, even on the same slab of limestone, there is some presump- 

 tive evidence that they should be referred to the same specific type, espe- 

 cially as there is only one kind of head and one kind of pygidium known 

 from the Park. The fact that both portions are without ornamentation 

 supports this view. The disparity in size, especially when viewed in 

 connection with a similar condition of affairs in Utah, is opposed to it. 



One of three hypotheses seems probable. Both species (and the 

 pygidia show that they are two) may have had unornarnented cephalic 

 shields, similar in detail and size; or, since it is now known that the two 

 species occur in the same beds, Hall and Whitfield may have been in error 

 in referring the cephalon from Dry Canyon to the associated pygidia; or, 

 the smooth, plain heads do not belong to the smooth pygidia with which 

 they are associated in the Yellowstone National Park, but (what is not 

 intrinsically improbable) to the ornamented nodose pygidia witli whicb 

 they occur at Dry Canyon, as Hall and Whitfield have suggested. More 

 evidence will be necessary before the point involved in this uncertainty 

 can be ascertained. 



Formation and locality: Madison limestone, Crowfoot Ridge, Grallatin 

 Range, bed 31; J. P. Iddings and W. H. Weed. Waverly age, Logan 

 Canyon, Wasatch Range, Utah. 



