HASTODOXSAr/RlD^. 



141 



Some writers divide this group into the two suborders of Stereo- 

 spondyli and Temnospondyli, according as to whether the vertebrae 

 are fully or imperfectly ossified, but the presence of rhachitomous 

 vertebras in the young of Mastodonsaurus seems to render such 

 division inadvisable. 



Family MASTODONSAURID^E. 



Skull triangular, and more or less elongated, with the cranial bones 

 very strongly sculptured, the occipital condyles ossified, and large 

 palatal vacuities (fig. 32) ; dentine of teeth with very complex 

 plications ; no bony ring in sclerotic ; and no ventral scutes. 

 Bodies of vertebrae fully ossified in the adult. 



There are large palato-vomerine tusks on the inner side of the 

 maxillary teeth ; and the palatines run parallel to the maxilla. 

 The mandible has a large postarticular process ; and there is a small 

 inner series of mandibular teeth. In the type genus the pubes are 

 separate from the ischia, and do not enter into the formation of the 

 acetabulum ; and the sacral ribs form kidney-like disks. 



the wedge-bones of Sphenodon ; this type 

 of structure being known as the embolo- 

 merous. In the trunk-vertebra of other 

 genera like Trimerorhachis (fig. 46) and 

 Archegosaurus each vertebra (fig. 30) con- 

 sists of four portions — viz., a basal inter- 

 centrum (hypocentrurn), a pair of pleuro- 

 centra, and a neural arch. In this rha- 

 chitomous type Cope regards the pleuro- 

 centra as representing the centrum of the 

 embolonierous type, since they both carry 

 the arch ; and as he finds that the func- 

 tional centra in other forms, like Chely- 

 osaurus, apparently correspond to the 

 intercentra of Archegosaurus, while the 

 pleurocentra are small and apparently 

 about to disappear, it is argued that in 

 other Amphibia the real centra are totally 

 wanting, and the vertebral bodies, which 

 in the caudal region have the chevrons 

 united to them, are really intercentra, to 

 which the neural arches have been shifted. 



Pig. 30. 



Diagram of a rhachitomous verte- 

 bra ; from the front and left side. 

 I, pleurocentra; 2, intercentrum ; 

 d, neural spine. {After Frifcch.) 



