HATCHER: OSTEOLOGY OF HAPLOCANTHOSAURUS i 
In this manner the centrum of the vertebra is reduced to superior and inferior hori- 
zontal plates united by a vertical median septum or plate. At the posterior ex- 
tremity these plates expand into a deeply excavated disk which forms the cup for 
the succeeding vertebra while at the anterior extremity they unite to form the ball 
of the centrum. A cross-section of the centrum midway between the anterior and 
posterior extremities is irregularly I-shaped and somewhat suggestive of that of an 
I beam in structural materials. The inferior surface of the centrum is broad and 
flat, much expanded posteriorly and moderately expanded anteriorly where at a 
point a little back of the ball it gives rise to the cervical rib. The cervical rib is 
firmly codssified both with the centrum below, through the intermedium of a para- 
pophysis, and with the diapophysis above. There is a short anterior branch of the 
cervical rib and a longer posterior one. The latter stops short of the posterior ex- 
tremity of the centrum. It is proportionately broader and stronger than in Diplo- 
docus carnegii but decidedly shorter and less robust than in Brontosawrus excelsus. 
Seen in front this vertebra appears rather low, with broadly expanded cervical 
ribs and prezygapophyses. There is a single supraprezygapophysial cavity and two 
infraprezygapophysial cavities separated by a median septum formed by the union 
of the horizontal laminze of opposite sides and supported below by the superior wall 
of the neural canal. In the vertebra under consideration the greater portion of this 
septum has been lost. It is restored in plaster, and in the drawings the restored 
parts are indicated by broken lines in the shading. As shown in the drawings the 
neural spine is also absolutely simple instead of deeply bifurcated as are the spines 
of the vertebree of this region in all other known genera of Sauropod dinosaurs 
wherever it has been possible to determine their character. The neural canal is 
rather large as compared with that in Diplodocus. 
Seen from the rear the neural canal is nearly circular and appears as if sunk into 
the superior surface of the centrum. The postzygapophysial laminee each send for- 
ward a broad thin plate. These unite with the neural spine and enclose a very deep 
suprapostzygapophysial cavity while below as in front there are two small but deep 
infrapostzygapophysial cavities separated by a median septum. 
The diapophyses are only moderately expanded and they are braced antero- 
posteriorly by the horizontal laminze and inferiorly by the inferior branches of the 
 diapophysial laminze which are very short and almost perpendicular. There is no 
superior branch of the diapophysial lamina. The posterior branch of the horizontal 
lamina runs obliquely upward and backward from the diapophysis to the posterior 
zygapophysis, thus giving additional support to the latter element. Another lamina, 
horizontal in position but homologous with one of the oblique lamine, runs directly 
