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the one formation to the other. Champsosaurus, belonging to another 
order, is found in the beds of the Lance Creek region and at Hell Creek 
and also in the Puerco; 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 Puerco, 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 Puerco 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, 
ELubaéna, Thescelus, Basilemys, Helopanoplia) are not known to have 
passed from the Lance Creek level into that of the Puerco and Fort Union; 
and other genera (Alamosemys, Hoplochelys, Conchochelys, Amyda?) ap- 
pear to have had their beginning in the Puerco. 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 Puerco. 
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. 
