Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/796

Rh 752 AM P H I B I A In some Labyrinthodonts, the capitular and tubercular processes, divergent and subequal in some (probably the anterior) vertebrae, coalesce into one in other vertebrae; and the capitular division being shorter than the tubercular, transverse processes, like those of the middle thoracic region of the crocodiles, are thus produced. In the Peromela, there is a short capitular process, but the tubercular process is represented by a mere facet placed below the prezygapophysis. In the Anura, finally, the vertebras have only a single transverse process (possibly representing the coalesced double transverse processes of the posterior trunk vertebrae of such Urodela as Menopoma), which, in some of the vertebrae, may attain a great length. Hibs are present in a few Anura, in all Urodela, Peromela, and Labyrinthodonta ; and, in the last-named and some Uro dela, they attain as great relative dimensions as in other Vertebrate. But they are always vertebral ribs, no Amphibian being known to possess more than rudiments of sternal ribs. The atlas is never provided with ribs. In the Peromela, ribs are borne by all the other vertebrae, except the very hind most. In the long-bodied Urodela, on the other hand, they are restricted to a few of the anterior vertebrae. In the other Urodela they are usually confined to the proa-sacral and sacral vertebrae ; but, in some cases, one or two of the anterior caudal vertebrae have free ribs. The form of the proximal end of the rib corresponds with that of the transverse processes or process. &quot;Where this is double, the rib presents a fork, formed by the capitulum and tuberculum ; and when the capitular and tubercular trans verse processes are of equal length, the capitula and tuber- cula of the ribs are equal ; but when either of the former is shorter than the other, the corresponding part of the rib is longer. The Peromela have no sternum, and that of the Labyrinthodonta (if they possessed any) is un known. In the Urodela, the sternum never ossifies, and there is no trace of even a cartilaginous sternum in Proteus. In Menobranchus, there is a very small cartilaginous sternal plate, which sends lateral prolongations into two of the inter- muscular ligaments, representing rudimentary sternal ribs. In the Newts 1 the sternum becomes a broad and stout plate of cartilage, with a median, posterior, cristate, xiphoid process, and with articular surfaces on its antero-lateral margins for the reception of the coracoids. The sternum attains its highest development in the raniform Anura, the xiphoid process becoming elongated and dilated at its ex tremity, and more or less converted into bone, while calci fication of the body of the sternum itself may also occur. The first two vertebra; of Mcnepoma (x ;,) * l &amp;gt; atlr &amp;gt; 8 ; J* 7 &amp;gt; second rertebra; a, intercondyloid processor the atlas; i, the articular surfaces for the occipital condyles. The ribs of the second vertebra arc not represented. A. dorsal ; B, ventral ; C, lateral view. 1 See Parker On the Shoulder Girdle, pp. C3, 68. Ill the Urodela, the first vertebra always presents two slightly concave articular facets, the faces of which look outwards and forwards to adapt themselves to the occipital condyles. Between these facets, the dorsal moiety of the anterior face of the centrum gives rise to a process, which is little more than a ridge in Proteus, but in other genera becomes very prominent, and has a curious resemblance to the odontoid process of the axis vertebra of a bird. This &quot;intercondyloid&quot; process of the atlas sometimes (Amphiuma, e.g.} exhibits on each side, near its termination, an articular facet, which plays on a corresponding facet of the adjacent face of the occipital condyle. Mayer (Analecten, p. 10) was misled by the form of this process into the supposition that the vertebra to which it belongs is not the atlas, but the odontoid, vertebra. But there is a similar process of the first vertebra in the Hays, and the relations of the vertebra to the nerves show that it is certainly not the homologue of the axis vertebra of other Vertebrates. The first spinal nerve, which haa the distribution of the hypo-glossal of the higher Vertebrata, passes out of the spinal canal, either between the first and second vertebrae, or through a foramen in the arch of the first, in the Amphibia, which have no proper suboccipital nerve. This is a very curious circumstance, and requires elucidation by the study of development. In the Anura, the atlas has the same general form, but the median process is either inconspicuous, as in liana esculenta, or may be absent. Among the Labyrintlwdonta, the atlas of Mastodonsaurus only is known. It presents two concave facets anteriorly, separated for about half their length by a notch, which probably lodged a ligament. The atlas of the Peromela has the two characteristic facets for the occipital condyles, but the intercondyloid process is absent, and the anterior margin of the arch of the vertebra projects forwards towards the corresponding margin of the occipital foramen (JEpicrium). In those Urodela which possess posterior limbs (except Pro teus and AmpJmima), one vertebra, or sometimes two , are distinguished A from the rest as &quot;sacral&quot; by having stouter ribs, the outer ends of which abut against, and are united by ligaments with, the ilia. The Anura always possess a sacral verte bra (sometimes anky- losed with its predeces sor or successor), the transverse processes of which are often en larged, and sometimes greatly expanded, at their iliac ends. The characters of the sa crum of the Labyrintho- donta are not known. In the Urodela, the anterior caudal verte- brae, except the first, fi& 2 _ J/ ^ ;on!(7 rosteriol . (A) and ventlal have interior arcnes, (B) views of the sacral vertebra; (S.V.); SM 1, which, like the neural -s- 2 , &quot;acrairibs; y/.iiium; /s.ischium. arches, are continuously ossified with the centra; and the same condition obtained in the caudal vertebrae of the Labyrinthodonts. In the Anura, the caudal vertebrae are replaced by a long coccyx, consisting of an osseous style, to the dorsal aspect of the anterior end of which two neural arches are