568 BRITISH FOSSIL REPTILES. 



narrowest part, is compactly dense ; there is a small medullary cavity (fig. 5) which 

 seems to have but a short longitudinal extent. 



A deep anconal depression (ib., fig. 2, i), marks that aspect of the distal expansion in 

 a greater degree than in any Crocodilian, Lacertian, Dinosaurian, or Pterosaurian 

 humerus that, as yet, has come under my notice ; it gives to this part of the humerus of 

 Omosaurus something of a Mammalian character. 



The following are admeasurements of the humerus : 



Length ..... 



Breadth across radial or pectoral crest 

 „ ,, distal end 



„ middle of shaft 



Girth of „ , . . 



Length of base of radial or pectoral crest 

 ulnar crest 



The figures of tliis bone on PL 70 are reduced to one fourth of the natural size. 



Although I should have hesitated to found a genus or generic term on a solitary 

 limb-bone if sucli distinction had not been supported by the vertebral characters, yet the 

 features were so much more strongly marked in the present than in previously described 

 or figured humeri as to have afforded a better excuse for such taxonomic deduction, which 

 ought to rest, and, as a rule, can only safely do so, on characters afforded by associated 

 parts of the skeleton or teeth. 



Mutilated as are the humeri discovered with unquestionable vertebrae of Cctiosaurus 

 longus in the Geological Museum of Oxford, justifying the conclusion that they belonged 

 to the same individual, they are unmistakably distinct in character from that bone in 

 Omosaurus. 



Although the radial or pectoral ridge be broken away in the subjects of figs. 4 and 5, 

 p. 585 {Cctiosaurus), its base has a minor relative extent than in Omosaurus ; the shaft 

 beyond that ridge expands more gradually into the distal end ; the entire length of the 

 bone — 4 feet 4 inches in Cetiosaurus longus — is greater in proportion to the breadth or 

 thickness of the shaft. 



The slender character of the humerus is more marked in that bone which chiefiy 

 represents Mantell's genus Pelorosaurus (' Dinosauria,' PI. 49), in which the radial or 

 pectoral crest (ib., fig. 2, d) subsides above the middle of the shaft, encroaching, as in 

 the Crocodile, Varanus, and Pterodactyle, upon the palmar surface of the bone. The 

 humerus of lyuanodon {' Di/iosauria' PI. 19) is still less robust in proportion to its 

 length, not to mention its inferior size as compared with associated dorsal vertebrae, than 

 in Omosaurus. 



