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Show 261 MR. ST. GEORGE MIVART ON THE [Apr. 28, CENTRA, OR VERTEBRAL BODIES. The bodies of the vertebrae unite together by the entire circumference 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 transverse 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-vertebrae oi Menobranchus, more than that in those of Amblystoma, and very much more in those of Proteus. Sometimes, as in Amblystoma 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 presents 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 vertebrae being biconcave. Such vertebrae are found in Siren, Proteus, Menobranchus, Menopoma, Amphiuma, Amblystoma, Plethodon, Aneides, & e On the other hand, in Salamandra, Triton, Pleurodeles, Chioglossa, Taricha, Spelerpes rubra, & e the part answering to the anterior cup is not only ossified, but ossification extends continuously and more or less thoroughly into the soft structure filling the cup of the centrum 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 vertebrae 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 laminae 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 laminae (that is, along the summit of each neural 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-vertebrae of Salamandra, Proteus, and most others. On the other hand, it is a very elevated process in the trunk-ver- |