244 FIELD COLUMBIAN MUSEUM. 
with all laws of organic development. We must, therefore, conclude 
that the two forms have arisen from a common ancestry, probably 
somewhere in the lowermost Jurassic, and that while the Haplocantho- 
saurus phylum remained in general conservative, it reached a high 
degree of specialization in this one particular. Brachiosaurus, on 
the other hand, is a somewhat later representative of the more highly 
specialized line which had taken to purely terrestrial habits. 
These conclusions are in accord with the evidence to be derived 
from the relative age of the horizons in which these fossils are found. 
While there is little reliable stratigraphic data for comparison be- 
tween the eastern and western Colorado localities, the evidence 
which has been adduced would indicate that Brachiosaurus comes 
from a horizon at least one hundred feet above the Cafion City 
quarry. However, various species of Apatosaurus and Morosaurus 
are found in the same horizons with both forms, so that the differ- 
ence in time alone is not sufficient to account for the differences 
between these two related forms. We must, therefore, attribute 
these differences chiefly to digressive development. 
PROBABLE HABITS. 
The habits of the Opisthoccelia have been regarded as semi- 
aquatic or at least marsh-dwelling. This conception was based 
largely upon the structure of the teeth, which are fitted for masticat- 
ing soft, succulent vegetation, and upon the ponderous bulk of the 
animals which seemed best suited for aquatic locomotion. It is true 
that the massive structure of the vertebre in such forms as A pato- 
saurus ajax bears some resemblances to the vertebre in cetaceans. 
However, the writer has failed to discover in the skeletal structure of 
aquatic or semi-aquatic animals, either reptilian or mammalian, any 
of that fluting and hollowing of the vertebrz which have been inter- 
preted as evidence of aquatic habits in the Opisthoccelia. This same 
effort at lightening the skeleton was regarded by an earlier English 
writer as evidence of aérial habits! 
That which appears to the present writer to afford most reliable 
evidence as to habits is the structure of foot and limb. As pointed 
out by Hatcher, there is no evidence among opisthoccelians of that 
shortening or angulation of limb, or the broadening of foot, which is 
common to amphibious animals. Nor is there anything in the struc- 
ture of the opisthoccelians which is not found in some terrestrial 
forms. The straight hind leg occurs in quadrupeds only among those 
forms which inhabit the uplands. Familiar instances of parallel 
development of this character are found in the proboscidea, dinocerata, 
