1 N V ERTEBR ATE PALJE ONTOLOGY. 



403 



Fie. r>6. 



"Whether or not these few intermediate types warrant the conclusion 

 Fig- 55. that the two forms merely rep- 



resent different sexes, or varie- 

 ties of one species, is a question 

 in regard to which there will 

 probably be different opinions. 

 In view of the fact, however, 

 that the compressed form has, 

 so far as I am aware at this 

 time, probably only been found 

 in the far West, where it some- 

 times occurs with the ovalus 

 form, and in other cases where 

 the latter has not been found, 

 I am not yet satisfied that they 

 do not rejjresent two closely- 

 allied, but distinct species, with 

 a tendency of their varieties to 

 blend together. 



Locality and position. — 

 Say's type-specimens of B. 

 comjwessus were from the 

 Upper Missouri, where they 

 were found near, or at, the 

 Great Bend below Fort Pierre, 

 in the Fort Pierre group of the 

 Upper Missouri Cretaceous 

 series. A part of our speci- 

 mens came from the same 

 locality and position ; while we 

 have many others from higher 

 positions in the same forma- 

 tion on Sage Creek, under the 

 Tertiary at the Bad Lands, and 

 on Cheyenne River; also, from 

 near the mouth of (lie latter, in Dakota. It likewise occurs near Milk River, 



BacuWes compressus. 

 Fig. 55. Outlines showing forms of sections at different stages 



of growth. 

 Fig. 56. Part of the non-septate portion oi a large specimen, 



showing lateral undulations. 



