466 CHARLES C. MOOK 
The reptilian fauna of the Morrison suggests the following 
analysis: 
a) Rhynchocephalia. The only modern representative of this 
group is non-marine, and there is nothing in the structure of the 
single Morrison representative of the order to suggest that it was 
anything different. It may perhaps have been amphibious, or 
fluviatile, or terrestrial. It is not probable that there was any 
direct competition between the members of this group and the 
sauropods. 
b) Crocodilia. Several species of mesosuchian crocodiles are 
known to have existed in Morrison time along with the Sauropoda. 
The modern crocodiles are amphibious creatures, either fluviatile 
or lacustrine, not marine, and the Morrison forms probably lived 
in a similar manner. They were good-sized, active, carnivorous, 
relatively intelligent animals, which may easily have preyed upon 
the young of the Sauropoda. 
c) Pterosauria. The pterodactyls were aérial forms. One 
species is known from the American Morrison. It is hardly to be 
expected that sauropod dinosaurs and pterosaurs would enter into 
any direct conflict or competition with each other. 
d) Squamata. Lizards, snakes, and mosasaurs are entirely 
unknown from the Morrison. Some of them must have been living 
somewhere at the time, for lizards are known to have existed since 
the Triassic, and mosasaurs appear well developed early in the 
Cretaceous. Consider in this connection the fact that the mosa- 
saurs were marine animals. 
e) Chelonia. Turtles at the present time are marine, fluviatile, 
lacustrine, or terrestrial. Only one species in this group is known 
from the Morrison, and it is probable that the chelonian ele- 
ment in the Morrison fauna was not very important. The 
turtles could hardly have come into severe competition with the 
Sauropoda. 
jf) Sauropterygia and Ichthyopterygia. The plesiosaurs and 
ichthyosaurs were entirely marine. No trace of them has been 
found in Morrison rocks, though they must have been living in the 
sea during Morrison time. They are found in marine rocks both 
younger and older than the Morrison. 
