398 THE ANIMAL KINGDOM. 



inferior arches of the tail are similarly conformed. These latter arches are 

 present also in Urodela, and are homologous with the neural arches and 

 similarly developed. The centra of the vertebrae are formed by ossification 

 of the cellular sheath of the notochord. They constrict the notochord 

 zWr#-vertebrally, while it is also constricted mfcr-vertebrally by a growth of 

 cartilage. If the inter- vertebral cartilage is little developed the vertebrae 

 are biconcave, as in Ichthyoidea or extinct Stegocephali, and some 

 Salamanders. But in the majority of the latter the cartilage is large in 

 amount and the inter-vertebral constriction great ; and in the highest forms, 

 e.g. Triton,\\. is segmented, forming opisthocoelous vertebrae, whilst theintra- 

 vertebral chorda is replaced by marrow. The inter- vertebral cartilage grows 

 rapidly in Anura, and the chorda persists for a long time intra-vertebrally ; 

 the centra are as a rule procoelous, but in a few, e.g. Pipa, Discoglossus, 

 opisthocoelous, or even variable, as in Pelobates. In these exceptional 

 instances the centra are formed for the most part above the notochord, not 

 round it, i.e. are epichordal as opposed to perichordal. The vertebral 

 column terminates in many Urodela with an imperfectly segmented carti- 

 laginous rod. In some Stegocephali a vertebral centrum, devoid of neural 

 arch and transverse processes, is intercalated between every two vertebrae 

 formed on the ordinary type. In others the centra are stated to consist of 

 a ventral piece (inter- or hypo-centrum) and two lateral pieces (pleuro- 

 centrum or centrum proper). These two forms have been distinguished as 

 embolomere and rhachitome. They occur, however, apparently in the 

 vertebral column of one and the same animal, the former in the tail, the 

 latter in the thorax, and therefore are not of classificatory value. And it 

 is not certain that the hypo- and pleuro-centra belong to the same vertebra 

 (Fritsch). The neural arches of some Stegocephali are complex (Branchio- 

 saurus). Ribs are present except on the cervical vertebrae and posterior 

 caudal. In Anura they are very small, united to the transverse processes, 

 and traces of a suture are rarely observable, e. g. Discoglossus. They are 

 bifurcated proximally in Urodela, a process articulating with both upper and 

 lower transverse processes, and are developed by the union of a short dorsal 

 and longer ventral rod. They do not surround the body, and in the genus 

 Spelerpes, among Salamandrina, are absent on two posterior thoracic 

 vertebrae, hence truly lumbar. The extreme shortness of the ribs in living 

 Amphibia is perhaps due to atrophy. They are long in some Stegocephali, 

 but there is no evidence of a ventral union. 



The sternum or hyposternum (see p. 82) in Urodela consists of a 

 shovel-shaped plate of cartilage formed by the union of two cartilaginous 

 rods lying in tendinous intersections of the recti abdominis muscles, with 

 cartilage derived from the coracoid. It is deeply grooved on either side for 

 the reception of the epicoracoidal edges of the coracoid. It has a somewhat 

 similar structure and origin in some Anura, e. g. Bombinator, but in the 



