ORDER I 



STEGOCEPHALIA 



119 



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 ArcJiegosaurus 



(Fig. 197). 



The rhachitomous type of vertebrae passes 



over into the embolomerous when the pleuro- 

 centra unite with the hyjDocentrum 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 

 (Cricotits) have rhachitomous dorsal, and embolomerous 

 caudal vertebrae. 



The most advanced stage of vertebral ossification 

 occurs among the Labyrinthodonts, where the centi'a are 

 solid bony discs, slightly amphicoelous ; sometimes they 

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



Fig. 197. 



Rhachitomous caudal vertebrae of 

 Archegosaurus. Lettering as in Fig. 196. 



FiCx. 198. 



Enibolomerous vertebrae 

 of Diplovertehron punctaium, 

 Fritsch. LoAver Permian ; 

 Bohemia, i/j (after Fritsch). 



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 

 m^y 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 diff"ei^ent 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 i-o.mmma aiimam, 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. 199. 

 Dorsal vertebrae of 



