HATCHER: OSTEOLOGY OF HAPILOCANTHOSAURUS 33 
less deep than in the last cervical of this series or the first dorsal of H. priscus. As 
in the succeeding dorsals the distance between the anterior and posterior zygapoph- 
yses is much abbreviated. There is a very faint emargination at the apex of the 
neural spine with a depth of only 7 mm. 
The Third Dorsal (Plate II., Fig. 3, Series 2).— This vertebra lay on end as 
shown at 12 in the first diagram. Owing to the position in which it lay in the 
quarry its centrum was much shortened by the pressure to which it was subjected, 
the ball having been forced down into the pleuro-central cavities and the whole 
centrum telescoped as it were. Fortunately the neural arch and spine are in a splen- 
did state of preservation. They are still held in position with the centrum, though 
the sutures are very distinct. The spine when compared with that of the preceding 
vertebra is much modified in the direction of the conditions that obtain in the suc 
ceeding dorsals. It has assumed a nearly vertical position instead of being inclined 
forward as in the preceding dorsals and cervicals. 
It is very much compressed antero-posteriorly and is still connected with the 
posterior zygapophyses by the superior branches of the post-zygapophysial lamine. 
In the present vertebra however this lamina does not run obliquely upward and 
backward in a direct and straight line from the zygapophysis to the top of the neural 
spine as in the preceding dorsals and the cervicals, but it extends backward, rising 
but little until it reaches the vertical plane of the anterior surface of the spine when 
it rises vertically as a thin narrow lamina ascending to the apex of the spine. The 
degree of differentiation in the neural spines of this and the immediately preceding 
vertebra is the most marked of any of the vertebree even in this region where the 
characters of the different vertebree are seen to change so rapidly. The apex of this 
spine is also faintly emarginate, the notch having a depth of 9mm. The position 
of the capitular rib facet is at the supero-anterior angle of the pleurocentral 
cavity. 
The Fourth Dorsal (Plate I1., Fig. 4, Series 2).—'The centrum and spine of this 
vertebra lay as shown at 13 and 13’ in the diagram. They are both well{preserved, 
and the nature of the spine and transverse processes demonstrate beyond a reason- 
able doubt that its position in the vertebral column was immediately posterior to 
the vertebra just described. The spine is now quite perpendicular and more ele- 
vated. It is much compressed antero-posteriorly but somewhat expanded trans- 
versely. Its anterior surface is transversely convex, the posterior is concave, form- 
ing a long, shallow trough or scoop not nearly so deep as in the preceding vertebree. 
The transverse processes in this and the immediately preceding vertebra are assum- 
ing a more elevated position, the neural arches are becoming higher and the trans- 
