118 



AMPHIBIA 



CLASS II 



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

 rod-shaped (Fig. 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 genera have it completely 



A 



2b i — 



.V 



Fig. 194. 



Phyllospondylous vertebrae of Branchiosaiirus 



arnhlystomus, Credner. Enlarged (after Credner). ch, 

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

 transverse process ; z, Zygapophysis. 



Fig. 193. 



A, Ventral armour of Branchiosaiirus. B, Scutes of 

 Branchiosaiirus; C, of Hylonomus; D, of Pelosaurus ; 

 E, of Archegosaurus ; F, of Sclerocephalus ,■ G, of Disco- 

 saurus ; H, of Petrdbates (after Credner). 



Fio. 105. 



Lepospondylous vertebrae of Hylonomus. ch, Noto- 

 chord replaced by mineral matter ; k, Bony cylinder 

 of the centrum ; c, Rib (after Credner). 



interrupted by ossification of the centra ; in the Branchiosauria it is com- 

 pletely persistent. 



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



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 Leposponch/li (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 the 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 



Archegosaurus. sp, Spinous process: i i • , , c . i -i , 1 



z, z', zygapophyses ; n, Neural arch- halves remain separate at first, but later coalesce 

 gw^Hypocentrum • pic, pieurocen- anc ] 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- 



Fig. 196. 



