264 ; MR. ST. GEORGE MIVART ON THE [Apr. 28, 
CENTRA, OR VERTEBRAL BopIeEs. 
The bodies of the vertebree unite together by the entire circum- 
ference of each of their juxtaposed surfaces, and sometimes by those 
surfaces also, according to the form and condition of the latter. 
Each vertebral body consists of a small bony cylindroidal structure, 
the antero-posterior extent of which considerably exceeds its trans- 
verse diameter, which latter about equals its vertical dimension. 
The length is generally less than twice the breadth ; it is about that 
in the mid trunk-vertebree of Menobranchus, more thau that in those 
of Amblystoma, and very much more in those of Proteus, Some- 
times, as in dmb/ystoma and notably in Proteus, the centrum is 
greatly constricted towards its middle, so as to appear to consist of 
two cones joined together at their apices, like an hour-glass. The 
posterior end of each vertebral body is always unossified, and pre- 
sents a bony concavity filled with gelatinous tissue. Very often the 
anterior surface of each centrum, except that of the cervical vertebra 
(hereafter described), is also similarly conditioned, the vertebre being 
biconcave. Such vertebree are found in Siren, Proteus, Menobran- 
chus, Menopoma, Amphiuma, Amblystoma, Plethodon, Aneides, &c. 
On the other hand, in Salamandra, Triton, Pleurodeles, Chioglossa, 
Taricha, Spelerpes rubra, &c. the part answering to the anterior cup 
is not only ossified, but ossification extends continuously and more 
or less thoroughly into the sott structure filling the cup of the cen- 
trum next in advance; so that each vertebral body comes to have 
a rounded convexity projecting from its anterior surface, and thus to 
articulate with its neighbour’s by an opisthoccelous ball-and-socket 
joint. 
Proccelous vertebrze do not exist in any species of Urodele. 
The most posterior centra are sometimes in the form of small 
rings, the central part of each being unossified and filled with a softer 
tissue. The centra are commonly (fig. 14) perforated below by small 
openings for blood-vessels. A centrum may be continuously ossified 
-with a neural arch above, a transverse process on each side, and 
hypaxial processes in an hypaxial arch below. 
NEURAPOPHYSES. 
The laminz which ascend on each side to embrace the spinal cord 
always, in the fully adult condition, unite together above and form 
a complete neural arch. In the Axolotl, however, the neural arch 
of the first vertebra (the cervical vertebra) long remains medianly 
cleft ; and in the Urodela generally a more or less deeply extending 
median notch very frequently divides the posterior part of each neural 
arch ; this is particularly marked in Siren (fig. 2). At the point of 
junction of the neural laminz (that is, along the summit of each neu- 
ral arch) a more or less marked antero-posteriorly directed ridge is 
usually to be detected. This ridge is sometimes very little developed, 
as in the trunk-vertebree of Salamandra, Proteus, and most others. 
On the other hand, it is a very elevated process in the trunk-ver- 
