STEGOCEPHALIA. 



137 



In the caudal region both the pleurocentra and hypocentra are complete 

 rings, and the neural arch is jointly supported by one of each of these 

 elements ; while the haemal arches (chevron bones) are borne exclusively by 

 the hypocentra. 



Mastodonsaurus (figs. 89, 90, 91 A). This is the largest known Laby- 

 rinthodont, the skull of M. giganteus attaining a length of l'25m. At 

 least one specimen has been discovered with the greater portion of the 

 trunk in natural association with the skull. The skull (fig. 91 A) is depressed, 

 a little longer than broad, with the orbits behind its middle. The external 

 bones are sculptured and bear conspicuous marks of the mucus-canal 

 system. The basioccipital region (fig. 89) is ossified and exhibits a pair of 



FIG. 89. 



Mastodowaurus giganteus ; hinder view of occiput, about one-sixth nat. size. 

 U. Triassic ; Wiirtemberg. b.occ. basioccipital ; c, occipital condyle ; 

 ex.occ., exoccipital; pas., parasphenoid; pt., pterygoid ; q.j., quadrato- 

 jugal ; qu., quadrate ; s.occ., space occupied by cartilaginous supraoccipital ; 

 s.t., supratemporals and (the large lateral plate) prosquamosal ; x, process 

 of exoccipital. (After E. Fraas.) 



large condyles (c.) for articulation with the vertebral column. The exocci- 

 pitals (ex. occ.} are also ossified, but do not enter into the formation of the 

 articular condyles ; while the supraoccipital must have been cartilaginous, 

 as it is represented in the fossils by a vacant space (. occ.}. The palatine 

 vacuities are very large, while the toothless pterygoids and paraspheuoid 

 exhibit their usual development. The palatine lies extended parallel to 

 the maxilla on each side and bears a single longitudinal series of teeth as 

 large as those on the margin of the jaw. The paired vomers, partly sepa- 

 rated by the parasphenoid, exhibit two pairs of relatively-large tusks, one 

 near the hinder end, the other in front of the posterior uares. The pre- 

 maxillfe are pierced to accommodate a pair of much-enlarged teeth at the 

 symphysis of the mandible. The mandibular rami are connected by 

 suture at the symphysis, and there is a coronoid process on each side, as 

 also a short post-articular extension ; but the limits of the component 

 elements are still very uncertain. The infoldings of the wall of the teeth 

 are extraordinarily complex (fig. 90). The vertebra are short, biconcave, 

 and pierced by a remnant of the notochord, while all except the atlas 

 (perhaps also except the hinder caudals) bear a pair of ribs and a loosely- 



