Professor Owen on BriiUk Fomil Reptilee. 71 



br«e, as compai'ed with the eocene and existing crocodiles, 

 could not have been anticipated ; and even now that they 

 are ascertained by repeated observation, some of them still 

 remain inexplicable in relation to any conjectural habits 

 of the species, or hypothetical conditions under which they 

 actually existed. We may understand why the ball and socket 

 articulation of the vertebra? of the present amphibious croco- 

 diles frequenting dry land, should be exchanged for a joint 

 having elastic substances included between two concave ar- 

 ticular surfaces, as a structure better adopted to crocodiles 

 more habitually living and moving in water ; but these croco- 

 diles with biconcave vertebrae are associated with others hav- 

 ing plano-concave vertebrae, and also with a species having 

 vertebrae joined by ball and socket articulations. And the 

 difficulty is not diminished by the remarkable fact of the lat- 

 ter structure being the reverse of that in recent crocodiles, 

 the ball and the cup having changed places in the extinct 

 S(reptospondi/lus ; and having assumed the position which 

 they present in certain sauroid fishes, and in the dorsal and 

 cervical vertebrae of some of the herbivorous mammalia. 



The biconcave, plano-concave, and convexo-concave modi- 

 fications of the vertebrae, are not the only points in which the 

 extinct crocodilians of the Wealden strata differ from those of 

 the London clay and from the existing species. The genus Go- 

 niopholiSf for example, exhibits a remarkable development of its 

 dermal armour, the large quadrangular scutes of which, inter- 

 locked by teeth received into depressions, are gigantic repre- 

 sentations of the scales of some of the Ganoid fishes ; while 

 the smaller hexagonal and pentagonal scutes* were articu- 

 lated together by marginal sutures, as in the dermal bony 

 covering of the armadillo. The Foikilopleuron exhibits a 

 medullary cavity in the body of the vertebrae, and a double 

 origin of the spinous process. The Suchosaurus offers a very 

 striking change in the form of the teeth. The Cetiosaurus 

 surpasses all modern crocodiles in its enormous bulk, which 



* These have been discovered since tlie first sheets of this Report vreni la. 

 press, by my friend Mr Holmes of Horsham, in the Wealden strata n^' 

 that toNvn. 



