294 



the one formation to the other. Cfoampsosaurus, belonging to another 

 order, is found in the beds of the Lance Creek region and at Hell Creek 

 and also in the Puereo ; but probably no species is common to the lower 

 and the upper levels. This genus, like Ptilodus, serves to show that, 

 though there may have been a considerable interval between the Lance 

 Creek and the Puereo, it was not' an enormous one. The dinosaurs, which 

 were such a conspicuous feature of the Lance Creek epoch, appear to have 

 disappeared completely before the time of the Puereo and Fort Union. 

 Of turtles, some families passed from the one formation to the other, but 

 probably no species. A pleurodire, representing a large group of turtles 

 found now mostly south of the equator, was present in the "Laramie" of 

 New Mexico ; but no member of the group is known to have existed in 

 North America after that time. Certain other genera of turtles (Adocus, 

 Euliaena, Thescelus, Basilemys, Helopanoplia) are not known to have 

 passed from the Lance Creek level into that of the Puereo and Fort Union ; 

 and other genera (Alamosemys, Hoylochelys, Conchochelys, Amyda?) ap- 

 pear to have had their beginning in the Puereo. It may further be said 

 that, while turtles were very abundant in the Lance Creek epoch, they 

 appear to have been very rare in the Fort Union, though of more frequent 

 occurrence in the Puereo. 



As regards the mollusks I find this statement made by Doctor Stanton 

 (Wash. Acad. Sci., xi, p. 264), where he is speaking of a Fort Union local- 

 ity in Montana : 



The Unios are all of simple type and do not include any of the pecu- 

 liarly sculptured forms like those of Hell Creek, Converse County, and 

 Black Buttes. 



The plants, conservative as they are, testify even more strongly than 

 do the animals to a considerable interval between the Lance Creek epoch 

 and the Fort Union. According to Doctor Knowlton (Wash. Acad. Sci.. 

 xi, p. 221), out of 84 identified species found in the Lance Creek epoch 

 ("Lower Fort Union") 68 occur in the Fort Union. Hence 16 species, 

 nearly 20 per cent, appear to have failed to reach the higher beds. It is 

 to be noted here that about 300 plants are known from the Fort Union 

 and only about 200 from the Lance Creek beds. For a group of organisms 

 that even then contained a considerable number of species yet existing, 

 or very close to forms yet existing, the loss of a fifth of their forces, at 

 a time when there appears to have been little change of climate, indicates 

 the lapse of an important interval. 



