REPTILES. 207 



which this class is distinguished from others, and particularly 

 from that of birds and mammals. For whilst in the tail-less ba- 

 trachians, only nine or eight vertebrae are counted, some serpents 

 have full three hundred. In most reptiles the body of the verte- 

 bra3 is convex on the posterior surface, concave on the anterior ; 

 in serpents especially these parts are to be regarded as true articu- 

 lar heads and articular cavities, whilst in them the vertebral 

 column has great mobility. The inferior spinous processes, which 

 exist in most of the vertebrae of the trunk in serpents, are curiously 

 modified in Rachiodon ( Tropidonotus scaber) : here these processes 

 of thirty or thirty-one vertebrae following the second are bent for- 

 ward, and the seven or eight posterior of them penetrate through the 

 oesophagus, and have enamelled point-like teeth 1 . 



The vertebrae of the batrachians have long transverse processes ; 

 ribs are wanting or are represented on some vertebrae merely by 

 cartilaginous excrescences at the extremity of the transverse pro- 

 cess. In the serpents the ribs are both very numerous and very 

 moveable; where they cease, the tail begins ; here the caudal 

 vertebrae may be distinguished from the other vertebrae of the 

 trunk, whilst all distinction of cervical, dorsal and lumbar vertebrae 

 fails. In the lizards such a distinction prevails, although the 

 lumbar vertebrae, and often the cervical vertebras also, are provided 

 with smaller ribs or rudiments of ribs. In the Chelonians, the ribs, 

 in conjunction with the spinous processes of the vertebrae, changed 

 into broad plates, form the dorsal shield (carapace), which covers 

 the body of these animals above. The ventral shield (plastron), on 

 the other hand, is formed by the large sternum, consisting of nine 

 pieces. In part, however, the dermal skeleton also enters into the 

 formation of these shields, by the marginal pieces on each side of 

 the dorsal shield, as well as by the unpaired dorsal plates, which 

 do not lie upon vertebral arches, and probably also by coalition 

 with the plate-like spinous processes of the vertebrae, and by am- 

 plification of the ribs and of the sternal pieces 2 . 



1 JOURDAN Institut. 1834, No. 60, 6r, DUM. et BIBRON Erpet. vi. p. 160 et suiv. 

 vii. p. 491 et suiv., STANNIUS Handbuch der Zootomie, ate Aufl., stes Heft, s. 20. 



2 After CARUS had expressed the opinion, that there is also an external skeleton, 

 a dermal skeleton, in the shield of tortoises which has coalesced with the neural 

 skeleton, the conception was more fully developed and defended by Dr W. PETERS, 



