460 CHARLES C. MOOK 
the group in question may aid in interpreting the structures and 
may guide us in determining the habits of fossil animals, because 
certain types of environment definitely exclude certain modes of 
life, as has been noted above. The habits of the animals must have 
conformed to the environment which actually surrounded them 
when they lived. 
The present discussion is concerned with the environment of the 
Sauropoda. The environment of a group of organisms is divisible 
into two components: first, the physical, and second, the biotic. 
These are related to each other in a complex manner, but with 
regard to their relation to a given group they may be considered 
separately. The first is concerned with such things as climate, 
with mountain, plain, delta, lake, or marine conditions, with the 
geographic extent of a given type of physical condition, with means 
of communication and barriers preventing communication or 
intermingling, with the rate of sedimentation and erosion, with the 
presence or absence of volcanic phenomena, etc. The second is 
concerned with food supply, with competition, and with enemies. 
If in a given case the various factors of each of these compo- 
nents may be determined, a comprehensive idea may be had 
of the habitat, or immediate environment, of the group of ani- 
mals involved. 
PHYSICAL ENVIRONMENT 
The physical environment of the Sauropoda is concerned with 
the geology of the Morrison, Arundel, Wealden, and corresponding 
Indian, African, Patagonian, and Malagasy formations. The 
American Morrison may be considered in this connection as an 
example of sauropod-bearing deposits. 
The physical characters of the Morrison, its relations to other 
formations, considered in connection with the general Mesozoic 
history of Western North America, indicate certain definite things 
regarding the geography, topography, climate, and dominant 
physical processes of the time and region in which the Western 
American Sauropoda lived. An extension of this study to include . 
world-wide conditions would give a fair idea of the physical 
environment in general. 
