228 KNOWLTON 



ing the aggregate number 49, as stated above. It is very probable, 

 however, that at least some of the undescribed Unios from Converse 

 County may prove to be identical with those from Hell Creek. It 

 is significant that of the 49 species only 4 species (8.3 per cent) are 

 common to the Laramie of the Denver (Colorado) region. These 

 are Anodonta parallela, Tulotoma thompsoni, Corbicula suhelliptica 

 and Viviparus plicapressus, the second species mentioned being 

 found also in the Montana, and two of the others at Black Buttes,^" 

 Wyoming, which reduces their value as diagnostic markers for the 

 Laramie. There are 8 species common to the "Hell Creek" and 

 "Ceratops beds," and the acknowledged Fort Union, of which 7 

 fall within the 18 species other than Unios. The most remarkable 

 feature of this fauna is the abundance of Unios, there being 31 

 of the 49 species that are referred to this genus, whereas in the Colo- 

 rado Laramie there is but a single fragmentary Unio, which cannot 

 be specifically identified. And, it may be added, these Unios are 

 found in the Hell Creek and Converse County areas in direct associ- 

 ation with both dinosaurs and plants, so there can be no question 

 as to their having come from within the dinosaur-bearing beds. 



Whitfield," who described the Unios from the Hell Creek region, 

 was so impressed with their evident affinity to species living in the 

 Mississippi and Ohio watersheds, that he gave to many of them 

 names designed to indicate this relationship {Unio gibbosoides 

 Whitf., cf. Unio gibbosus Barnes; U. retusoides Whitf., cf. U. reltisus 

 Lam., etc.), and adds: 



Some of them are so nearly like the living species that it would do 

 but little violence to specific features, to state that they were the same. 



This indicates either that the "Hell Creek beds "are comparatively 

 recent, or that Unios are not of great value as time markers. As 

 Brown well says''^ "The invertebrates plainly forshadow Tertiary 

 and living species." 



"The fossiliferous, dinosaur-bearing bed at Black Buttes, Wyoming, is 

 not here admitted as being certainly referable to the Laramie, but rather to 

 post- Laramie, or Shoshone. 



*• Bull. Am. Mus. Nat. Hist., vol. 19, 1903, pp. 483-487; idem, vol. 23, 

 1907, pp. 623-628. 



" Idem, vol. 23, 1907, p. 845. 



