THE BONES IN THE ENALIOSAURIA. 305 



while in Chelone it meets the quadrate ; in Iclithyosaurus it meets 

 the quadrate by curving under the squamosal. The splint cha- 

 racter is wanting from the Chelonian lower jaw ; it has no keel 

 behind the articulation ; and the dentary bone is single. 



The vertebral column is very dissimilar, there not being a single 

 character in common. 



The ribs, limited to the dorsal region in Chelonians, would seem 

 to be as dissimilar as the vertebrae, unless, indeed, the seemingly 

 double rib of the Ichthyosaur, grooved throughout its length, be 

 an epipleural growth repeating the costal rib upon itself through- 

 out its length, as is the case with Chelonians. 



The pectoral girdle is altogether dissimilar, being formed in 

 Chelonians of long spathulate coracoids which do not meet eacK 

 other mesially, and of rod-like scapulse which give off a long rod- 

 like precoracoid process. 



The pelvic arches are only similar in the form of the ilium, 

 which in both is a short curved bone rather compressed. The 

 two trochanters to the head of the femur of Chelydra, though not so 

 well developed and not opposite to each other, are homologous 

 with those of the head of the femur in Ichthyosaurus. Beyond 

 this there is in the limbs no character in common worth dwell- 

 ing on. 



§ 5. The Lacertian Characters o/" Ichthyosaurus. 



All living reptiles are with difficulty compared with Ichthyo- 

 saurus, owing to the backward and lateral position of its nares. 

 Polychrus anomalus has the nares far back ; and in the Nilotic Mo- 

 nitor they are not near the tip of the snout ; but in no lizard do 

 the premaxillaries form the lateral margins of the palate, as in 

 Ichthyosaurus ; and, contrary to the rule with Ichthyosaurs, they 

 are single and prolonged back mesially between the nares, except 

 when, as in Monitor, the nares reach so far back that they are 

 divided by the nasal as they are in Ichthyosaurus ; only in Mo- 

 nitor the nasal bone is single and narrow. Lizards differ in having 

 the whole lateral alveolar border made by the maxillary bones 

 margined by a conspicuous row of foramina. 



The orbit of Ichthyosaurus is, circumscribed by bones as in many 

 lizards, such as Iguana, Uromastix, Stellio, Scincus, Draco, &c. ; 

 but the bones which enter into its outer margin are not the same. 

 At the base in both is the long curved malar, and in front of that 

 in lizards a small, and in Ichthyosaurs a large lachrymal bone. 



