TUE BONES 15 THE ENALIOSAUKIA. 303 



rallj^ and iu the dorsal region give off strong compressed trans- 

 verse processes to which the ribs are exclusively attached; that 

 the centrum is procoelian, and in the caudal region furnished 

 with chevron bones ; that the articulation for the rib in passing 

 from neck to back in Crocodile ascends, while iu Ichthyosaurus it 

 descends. 



The ribs also have little in common ; for in Ichthyosaurus 

 they are of a generally uniform character, while in Crocodiles 

 there are double-headed j^-shaped cervical ribs, double-headed 

 dorsal ribs which never articulate with the centrum, and no caudal 

 ribs at all. There are no median sternal ribs. 



The pectoral girdle dilfers iu crocodiles having no clavicles or 

 interclavicle, and by the coracoids articulating with a narrow ster- 

 num which extends beyond them anteriorly and posteriorly. 



The scapula of the crocodile would have a general similarity of 

 form if its auterior margin were straight instead of being con- 

 cave ; but at the expanded end the surface for the coracoid would 

 have to be in front, aud that for the humerus behind ; and the 

 coracoids, besides a similar reversal of articular surface, swould 

 require immense antero-posterior expansion to change the hour- 

 glass shape of the crocodilian bone into the transversely pedicled 

 ovate shape of the bone in Icthyosaurus. 



In the pelvis the ilium is quite dissimilar ; and the crocodilian 

 ischium differs in supporting the pubis (?) on a pedicle so as to 

 exclude it from the acetabulum. And thus the pubes are thrust 

 forward, so that they do not meet the ischia in the line of sym- 

 physis ; and the crocodilian pubis never has the anterior margin 

 straight. The ichthyosaurian ischium never has its proximal 

 end so massive as in the crocodile. 



The functional elongation of the limbs in crocodiles in rela- 

 tion to movement on land gives to their several segments cha- 

 racters which make comparison with Ichthyosaurs impossible. 

 In the femur the only resemblances are that the articular ends 

 are compressed, the proximal one rounded and at right angles 

 to the distal end. 



§ 4. The Chelonian Characters ©/"Ichthyosaurus. 



On the upper surface of the skull the correspondence between 

 the two types is limited to the median bones of that roof of the 

 skull being double, the elongation of the parietal bones in har- 

 mony with large temporal fossfe, the vertical position of the orbits, 



