THE 130NES IN THE ENALIOSAUfilA. 315 



have little in common — the ilium being greatly expanded in Di- 

 nosaurs, while the pubis and ischium are more elongated than in 

 Ichthyosaurs, and leas expanded at the symphysial end. 



The proportions of the limbs are reversed, the hind limbs of 

 Dinosaurs being the larger. In the bones of the limbs there is no 

 correspondence of form or plan. 



§13. The Dicynodont Characters of lchi}\jo^di\a:\)L^. 



In Dicynodon the premaxillary and the parietal bones are single, 

 and the foramen parietale perforates the parietal only, so far being 

 unlike Ichthyosaurus. But the nares and orbits are lateral and 

 circumscribed, though the orbits are relatively smaller, and the 

 nares are, from the shortness of the snout, not so far back in the 

 skull. The temporal fossae are similarly circumscribed ; and the 

 parietals and squamosals similarly diverge behind, though often 

 with a more marked Y-shape in Dicynodonts than in Ichthyosau- 

 rus. The peculiar bones of the Ichthyosaur's skull are wanting ; 

 and the quadrate bone is a naked pedicle firmly united to the solid 

 vertical back part of the skull. The dicynodont occipital condyle 

 consists of three equal parts contributed by the exoccipital and 

 basi occipital bones. The palate and teeth are both unlike those 

 of Ichhyosaurus. 



The vertebrae are often not dissimilar to those of Ichthyosaurus 

 in the deep cupping of the centrum ; but the centrum is longer, 

 the neural arch is auchylosed to it, and the attachment for the ribs 

 is altogether different in Dicynodonts. 



The pectoral and pelvic arches are altogether dissimilar ; and 

 the limbs, except in the great expansion of the humerus at its 

 proximal and distal ends, have nothing in common. 



§ 14. The Lahyrinthodont Characters of Ichthyosaurus. 



Labyrinthodonts agree with Ichthyosaurs in having the me- 

 dian roof-bones of the skull all double, in having the orbits cir- 

 cumscribed with bones, with a postorbital bone behind, and a 

 supraquadrate bone behind that, between the squamosal and 

 quadrato-jugal. But they differ in having the temporal fosste 

 entirely roofed over, in the foramen parietale perforating the 

 middle of the parietal bones, in the orbits being small, in the 

 great elongation of the principal frontal bones, in the forwarc 

 position of the nares (usually nearer to the end of the jaw than to 

 the orbits), and in the short premaxillary bones (which sometimes 



