STRUCTURE AND RELATIONSHIPS OF OPISTHOCELIAN DINOSAURS. 243 
It will be observed that in Haplocanthosaurus the zygapophyses 
are about as strong as those of Morosaurus, and extend a like distance 
from the median line. Their articular surfaces are plane and directed 
slightly upward as well as laterally. The hyposphene has a moderate 
development. In Brachiosaurus, on the other hand, there is a 
marked crowding together of the zygapophyses at the median line, 
especially in the mid-dorsal region, and an unusual development of 
the hyposphene-hypantrum articulation. The articular surfaces of 
_the zygapophyses are directed laterally and are slightly curved, so 
that the postzygapophyses embrace the anterior pair. The centra 
of the dorsal vertebrz in the larger genus are, with the exception of 
the last, much more elongate and more thinly walled than in the. 
smaller form. The sacral ribs also show a much higher degree of 
specialization. In Haplocanthosaurus they arise uniformly from the 
anterior end of the centra and show but little variation in strength 
throughout the series. In the larger animal, however, the second 
pair are greatly expanded, as has been described above, while the 
third pair are correspondingly reduced in size. There is also a much 
greater relative expansion of the thoracic cavity, as is shown by the 
comparative length of the anterior ribs. 
The neural spines of Brachtosaurus, so far as preserved, present 
the same type of structure as do those of Haplocanthosaurus, but 
while those of the latter genus reach their greatest length at the third 
presacral and begin to diminish with the sixth, those of the former 
increase regularly to the seventh. There is also a more pronounced 
shortening of the neural spines and transverse processes of the 
anterior caudals in Brachtosaurus, indicating a reduction of the 
caudal series, consistent with the cephalad specialization of this form. 
In most of the above noted differences, Haplocanthosaurus presents | 
a more primitive structure, from which Brachtosaurus might well be 
derived. However, the elevation of the neural arches and the 
upward direction of the diapophyses in the smaller form present a 
degree of specialization which precludes this possibility. A very 
similar condition exists in Stegosaurus, probably as an adaptation to 
the support of its dermal armor. While it is hardly possible that 
such an armor could have existed in this form without having been 
preserved in one or the other of the two specimens described by 
Hatcher, the elongation may have resulted from a like muscular 
development, or it may be due to acquired aquatic habits, as such a 
tendency is found in certain cetaceans. To assume that the dorsal 
pedicles had attained such an elevation and were again reduced to the 
low, broad structure found in Brachiosaurus, would be inconsistent 
