118 



AMPHIBIA 



CLASS II 



in form, the more common varieties being oval, rhomboid, oblong, fusiform, or 

 rod-shaped (Pig. 193). 



The vertebral column usually remains in an embryonic condition, and 

 recalls that of Ganoid fishes. The degree of persistence of the notochord is 

 very variable, but only the most specialised gerj^ra have it completely 



Fio. 194. 



Phyllos]ioiidylous vertebrae of Branchinsmirus 

 amUystomns, Credner. Eularged(afterCretlner). eh, 

 Notochord ; n, Neural arch ; sp, spinous process ; d, 

 transverse process ; z, Zygapophysis. 



Fio. 193. 



A, Ventral armour of Branchiosaurus. B, Scutes of 

 Branchiosaurus ; C, of Hylonomus ; D, of Pelosaurus ; 

 E, of Arckegosailms ; F, of Sclerocephalns ; G, of Disco- 

 saurKs ; H, of Fetrobates (after Credner). 



Fig. 195. 



Lepospondylons vertebrae of Hylonomus. ch, Noto- 

 cliord replaced by niineral matter ; k, Bony cylinder 

 of tlie centruni ; c, Rib (after Credner). 



in the Branchiosauria it is com- 



interrupted by ossification of the centra 

 pletely persistent. 



Ossification of the vertebral column is least complete among the Fliyllo- 

 spondyli (Fig. 194), where, in the absence of 

 pleurocentra, the neural arch itself extends down- 

 ward and joins with a pair of delicate hypocentra 

 (or intercentra) to enclose the notochord and to 

 form transverse processes for support of the ribs. 

 In the Lepospo7idyli (Fig. 195) the notochord is 

 persistent and encased in constricted bony 

 cylinders, which are hourglass-shaped in longi- 

 tudinal section. In some genera, however, the 

 continuity of t.he chord is all but interrupted 

 by ossification of the middle portion of the centra. 

 The Temnospondyli (Fig. 196) have the ver- 

 tebrae composed of several distinct pieces. 

 Rhachitomous dorsal vertebrae of Ossification begins with the neural arch, whose 

 ^/J, zTgapophysTs;^7^TeLKrd^ halves remain separate at first, but later coalesce 

 tnun."^^'"''''"^'""'" '' ^'''' ^^^"'""'=^"' and are continued above into a stout and some- 



times distally thickened spinous process. The 

 centra may be either rhachitomous or embolomerous. Those of the rhachitomous 

 type are formed by a horseshoe-shaped basal piece (hypocentrum or inter- 



Fio. 196. 



