514 R. Owen, Esq., on the Dislocation of the Tail of the Ichthyosauri. 



and have received, through the kindness of Sir Philip Grey Egerton, a sepa- 

 rate terminal caudal vertebra of an Ichthyosaurus from Lyme Regis, all of 

 which exhibit a compressed form nearly as well marked, in comparison to the 

 vertebras of the rest of the spine, as is the depressed form in the terminal ver- 

 tebras of the Cetacea. This structure offers additional and very satisfactory 

 proof of the existence of a caudal tegumentary fin expanded in the vertical 

 direction. Such a fin would be especially useful in the short- and stiflf-necked 

 Ichthyosaur ; while in the Plesiosaur, where the length and structure of the 

 neck are so favourable for rapid lateral inflections of the head, an instrument 

 like that which produces the corresponding movements in fishes would be un- 

 necessary. Now, it is an interesting fact, that in all those skeletons of Ple- 

 siosaurs in which the tail is perfect, it is straight ; and there is no indication 

 of the partial fracture or bend in them, which is so common in the tails of the 

 Ichthyosaurs, 



March 18th, 1838. 



