HATCHER: OSTEOLOGY OF HAPLOCANTHOSAURUS 45 
considered as constituting a single group. Evidence of such antiquity is found not 
alone in the great diversity exhibited by the three subdivisions into which the 
group as a whole has been divided but by the diversity and specialization exhibited 
by the different families, genera and species within each of these three subdivisions, 
As yet we know comparatively little of the earlier Dinosawria and the group if in 
reality it be a natural one is at present represented in our museums for the most 
part only by the later and more specialized forms. Of the Sawropoda we know only 
those forms which lived just prior to their extermination when they were already 
highly specialized. Consider for a moment the enormous time interval which must 
have been necessary for the development of a reptile like Diplodocus. Yet his 
remains are found associated in the same quarry with those of Haplocanthosawrus. 
the most primitive Sauropod known, and the entire range of the Sauropoda through- 
out the geological column in North America so far as at present known is limited 
to certain horizons in the Jurassic or Lower Cretaceous of some authors, with a ver- 
tical thickness never exceeding a few hundred feet, and from the top to the bottom 
of which there are always found forms which are highly specialized, conclusive 
proof that the paleontological record is exceedingly incomplete as regards this 
group. 
Although the time distribution of the Theropoda and Predentata as we now know 
it is more considerable than that of the Sawropoda yet it is by no means complete 
and we know little of the earlier forms of either of these divisions. The wonder 
therefore is not that the three divisions as we now know them should show so little 
in common, but rather that, considering their great antiquity and early differentia- 
tion, they should have continued to possess in common even such characters as they 
do show. 
Seconp.— Although due weight should be given to every marked and important 
difference in structure it should nevertheless be borne in mind that every character 
‘possessed in common by these three divisions or by any two of them should be con- 
sidered as an evidence of relationship until definitely proved to be fortuituous or as 
having been developed independently in each instance. 
Turrp.—It isin the, as yet undiscovered, earlier and more generalized members 
of these groups that we must look for those characters which will throw most light 
on this question. If future explorations should be rewarded by the discovery in 
the early or middle Trias of a considerable number of representatives of each of the 
groups which we now refer to the Dinosawria, and if together they were shown to 
possess many characters in common and to approach one another much more nearly 
than do the Jurassic and Cretaceous forms, this evidence would be considered as 
