88 
FOSSIL REPTILIA OF THE 
The centrum is more or less antero-posteriorly compressed (PL XXII, figs. I to 7, 9, 
14), with concave terminal articnlar surfaces (ib., fig. 6) not intercommunicating; on each 
side of the shallow myelonal canal (PL XXII, figs. 2 and 4, m) is the deeper, usually 
triangular, articular surface {np) for the neurapophyses (?*). These, in each vertebra, converge, 
and, save in the atlas (PL XXI, fig. 1, n), coalesce at their summits with each other and 
with the neural spine (ib. figs. 2, 3, 4, n, hs). In most Fishes the neural arch coalesces 
with the centrum, as in Cetaceans ; its separate state is a saurian, chiefly crocodilian, 
modification ; it is such in the Ichthyosaurs, and adds to the power of inflecting the spine 
vertically, as in the specimen (PL XXIX, fig. 2).^ 
Most of the neurapophyses interlock by means of coadapted zygapophyses (PL XXI, 
fig. 6, s,s'). The heemapophyses are developed beneath the abdominal ribs (ib., 
fig. 2,/i,h\) and beneath the bodies of most of the caudal vertebrae (ib., figs. 4, 5, k) ; 
they are always distinct from their centrum (c), and do not coalesce below with each other, 
or with a haemal spine. The hypapophyses remain detached in the first two or three 
vertebrae (ib. fig. 1, hi/), and have advanced to the interspace between their own and the 
antecedent centrum. That of the atlas (PL XXIII, fig. 5, hya) is wedged between it and 
the basioccipital (o) ; that of the axis (ib., hyx) between it and the atlas, and so on (ib., 
hy, 3). CoNYBEAUE, wdio first iioticed this structure, describes it as follows : — We have 
only seen the inferior piece or body (if it can be so called) of the atlas ; and the odontoid 
process (which in all reptiles forms a distinct piece) of the axis ; they very nearly resemble 
those of the turtle.”" 
In the trunk the centrum of the atlas (PL XXIII, figs. 2 — 5, c a) is the most modi- 
fied of that series of vertebral elements. Its fore surface (ib., fig. 2) presents at its upper 
two thirds a concavity (c a), occupying the medial two fourths of its transverse extent, 
the cavity gradually changing to convexity (b) in the lateral fourth. Beneath this 
smooth concavo-convex articular surface is a rough, flat, triangular surface {t), inclining 
from its upper base backward. The upper joint-surface (c «), is for the basioccipital 
(PL XXIII, figs. I, 2, fig. 5, in dotted outline), the lower one {t), is for the hypapophysis 
(Fig. 5, kya)- The hind surface of the atlantal centrum (PL XXIII, fig. 4) is flat, and with 
^ More extreme and abrupt vertical flexures, shown in two specimens in the British Museum, may be 
posthumous, due to disturbance of the decomposing carcase prior to final hurial in the Liassic mud, in and 
with which the skeleton subsequently became petrified. 
2 ‘Trans. Geol. Soc.,’ vol. v, 1821, p. 574. The homology of the “odontoid process” as the 
“centrum of the atlas,” and that of the anthropotomical “body of the atlas ” with the hypapophysial part 
of that vertebra, is shown in my “Description of the Atlas, Axis, and Subvertebral Wedge-bones in the 
Plesiosaurus,” ‘Annals and Magazine of Natural History,’ vol. xx, 1847, p. 217, figs. 1 — 6. 
In 1835 Sir P. de M. Grey Egerton communicated to the Geological Society his discovery of not 
only Conybeare’s “ infei'ior piece of the atlas,” but the homotypal parts of the two succeeding vertebrae 
(‘ Proceedings of the Society,’ vol. ii. No. 41, p. 192), and suhsequently gave a detailed description, with 
figures, of these parts under the name of “ suhvertebral w'edge-hones ; ” ‘ Trans. Geol. Soc.,’ 2nd series, 
vol. V, 1836, p. 187, pi. xiv. 
