44 MEMOIRS OF THE CARNEGIE MUSEUM 
conclusive evidence as to its affinities with that group of the Dinosauria. Never- 
theless there are a few characters, such as the elongated neural arches and compara- 
tively short and absolutely simple neural spines of the dorsal and posterior cervical 
series, the conformation of the transverse processes and position of the capitular rib 
facets, which are different from any other known member of the Sauwropoda of North 
America and are more nearly paralleled by the characters which exist in the verte- 
bree of this region in some members of the Predentata, more especially in Stegosawrus, 
where, as in Haplocanthosaurus, the neural spines are short as compared with the ele- 
vated neural arches from the summit of which, in each case in the posterior dorsals, 
the transverse processes spring and diverge from the bases of the perpendicular 
spines at angles of about 45° instead of being directed horizontally as is the usual 
manner in the Sawropoda. The presence of characters so similar as those just men- 
tioned in representatives of the Sawropoda and the Predentata while certainly not 
indicative of any very close relationship may perhaps be considered as evidence of a 
remote common ancestry for the two groups. If this view be taken, these characters 
possessed in common would be considered not as parallel or analogous characters de- 
veloped independently in each instance, but as persistent primitive characters which 
were present in their remote but common ancestors. As the development of the 
two groups progressed and they became more and more differentiated, such charac- 
ters proved advantageous and became more emphasized in the Predentata while in 
the Sawropoda, where for some reason they were not particularly advantageous, they 
were gradually eliminated and disappeared altogether in the more highly specialized 
forms though persisting in the more primitive Haplocanthosaurus. Itis by the pres- 
ence in common, among the Sawropoda, Theropoda and Predentata, of such charac- 
ters as those just described, that the student of the Dinosauria will find the most 
trustworthy evidence as to the actual relationships or want of relationships in the 
three groups. Not until a considerable number of genera within each group are 
known from a detailed study of the osteology of fairly complete skeletons will it be 
possible to pronounce with any degree of certainty even upon the question as to 
whether the Dinosauria is a natural group as maintained by Marsh or an entirely 
unnatural one, without any right to existence, into which has been thrown three 
distinct groups, totally dissimilar and with nothing in common, as was held by the 
late Dr. George Baur. In the discussion of this question however there are several 
points which should be constantly kept in mind by the advocates of either view. 
Among these are: 
Frrst.— Those who are opposed to considering the Dinosauria as a natural group 
should bear in mind the great antiquity that must be accorded to that group when 
