THE BOIS-ES IN THE EKALIOSAURIA. 307 



character in passing backward, unlike Ichthyosaurus. The lower 

 jaw of lizards has a strong, vertically developed, coronoid bone, 

 not seen in Ichthyosaurs ; and the bones have not the usual splint- 

 like ovei'lapping. The dentary forms half of the jaw. 



In number of vertebrae and in length of tail lizards rival Ich- 

 thyosaurs. And in the Ilatleria the centrum is biconcave ; but 

 in all lizards it is greatly longer, and in most lizards the cen- 

 trum is procoelous, and in certain tail-vertebrse ossified in two 

 parts, anterior and posterior. In Monitor the neural spine is 

 vertical and quadrate, but not so long as in Ichthyosaurus, except 

 in the tail ; and there, relatively to the centrum, it is not so wide. 

 The neural arch is anchylosed to the centrum in lizards, and se- 

 parate in Ichthyosaurs. In the lizards there are often both trans- 

 verse processes and chevron bones in the tail, neither occurring in 

 Ichthyosaurus — though the transverse processes of reptilian caudal 

 vertebrae have the aspect of caudal ribs, like those of Ichthyosaurs, 

 anchylosed to the centrum. The ribs of lizards are supported on 

 a strong short pedicle, which appears to be contributed to by both 

 neural arch and centrum, and is at the anterior end of the ver- 

 tebra, well below the praezygapophysis ; •^'kWe in. Ichthyosaurus fke 

 articular thoracic tubercles are small, double, and raised but little 

 above the surface of the centrum. 



The costal ribs of lizards are strong, less compressed from 

 front to back, want the groove which runs along the middle of an 

 Ichthyosaur's rib, and have the proximal articulation massive and 

 single, instead of compressed and terminating in two articular 

 tubercles. 



The sternal and median ribs, unlike those of Ichthyosaurus, are 

 modified in relation to a sternum, are not well ossified, and do 

 not unite with the other costal elements by overlap. 



In the pectoral girdle there is the fundamental difference that 

 lizards have a sternum, but in spite of it the coracoids, by a 

 wide median expansion, almost meet mesially. Their approxima- 

 tions, however, are (typically) deeply emarginate ; and so the whole 

 bone becomes dissimilar in form to the coracoid of Ichthyosaurus, 

 though in Polychrus, for example, the coracoid is small and not 

 unlike that of Ichthyosaurus. Here, too, the scapula is more ich- 

 thyosaurian than usual with lizards, some, like Monitor, having the 

 bone united with the coracoid throughout its length, others, like 

 Iguana and Skink, giving off a strong acromion process from the 

 anterior margin ; but in Polychrus, Draco, &c. the bone is com- 



