ORDER I 



STENOCEPHALIA 



119 



Fig. 197. 



Rhachitomous caudal vertebrae i>t' 

 Archegosaurus. Lettering as in Fig. L96. 



centrum, the tapering extremities of which are directed upward), and a pair 



of lateral pieces, or pleurocentra ; a structure which is paralleled in certain 



Ganoids (cf. Fig. 121). The hypocentrum usually 



ossifies in advance of the pleurocentra, and lies 



directly underneath the neural arch. Occasionally 



a small horizontal basal piece is inserted beneath 



the pleurocentra and between the hypocentra. 



This piece, according to von Meyer, appears to 



be divided in the anterior caudals of Archegosaurus 



(Fig. 197). 



The rhachitomous type of vertebrae passes 



over into the embolomerous when the pleuro- 

 centra unite with the hypocentrum to form a 



complete bony ring, which coossifies with the 



neural arch above (Fig. 198). Or occasionally 



the crescentic intercentrum develops into a com- 

 plete ring, and the pleurocentra unite with each 



other below and the neural arch above to form a second ring. Some genera 



(Cricotus) have rhachitomous dorsal, and embolomerous 

 caudal vertebrae. 



The most advanced stage of vertebral ossification 

 occurs among the Labyrinthodonts, where the centra are 

 solid bony discs, slightly amphicoelous ; sometimes they 

 are pierced for the passage of the notochord (Fig. 199), 

 or a channel is visible below the neural canal through 

 which it extended (Stereospondyli). 



There is usually but one cervical vertebra, the atlas, 

 which is concave in front and facetted to receive the 



exoccipital condyles, but bears neither ribs nor anterior zygapophyses. Most 



or all of the presacral or dorsal series bear ribs, which 



may be either single or double-headed. The single sacral 



vertebra is provided with a pair of stout transverse pro- 

 cesses for the attachment of sacral ribs. Haemal arches 



(haemapophyses, " chevron bones") are borne by the caudal 



series, being joined to the centrum (pleurocentrum) or 



intercentrum. 



The cranium (Fig. 200) exhibits the depressed, broadly 



triangular form characteristic of amphibians, but is invested 



by bony plates which form a very different pattern from 



the usual amphibian type, while resembling in some 



respects those of Ganoids and Crocodiles. The cranial 



plates are usually ornamented externally with radiating 



sculpture, pittings, or punctae, and sensory canals are 



often conspicuous. 



The cranial roof is pierced by several apertures, the f° > Huxley 



largest of which are the orbits, and around them there is 



often a sclerotic ring. The anterior nares are placed close to the border of the 



snout, and separated from each other by a considerable interval. Another 



and smaller opening occurs at the median suture between the parietals, 



corresponding in position to the so-called parietal foramen of Lizards. 



Fig. 198. 



Embolomerous vertebrae 

 of Diplovertehr&n punctatum, 



Fritsch. Lower Permian ; 

 Bohemia, 1/] (after Fritsch ). 



Fig. L99. 



Dorsal vertebrae of 



