[ SH ] 



XXXVI. — Note on the Dislocation of the Tail at a certain point ob- 

 servable in the Skeleton of many Ichthyosauri. 



By RICHARD OWEN, Esq., F.R.S., F.G.S., Hunterian Professor to the 

 Royal College of Surgeons, London. 



Plate XLII. 



[Read March 21st, 1838.] 



If the Cetacea, like the Enaliosauria, were known only by their fossilized 

 skeletons, it can hardly be doubted but that their conjecturally-restored figures 

 as when entire and alive, would have resembled, so far at least as regards the 

 form of the tail, those which have been published of the Ichthyosaurus. It may, 

 I think, be safely affirmed, that the depressed or flattened shape of the small 

 vertebral centres which terminate the gradually tapering tail in the skeleton of 

 the Dolphins and Whales, would never of themselves have suggested the ex- 

 istence, in the recent and entire animal, of so large and important an instrument 

 of locomotion, due entirely to unossified and readily decomposable material, 

 which the actual presence of these fish-like Mammalia in the existing state of 

 things, places beyond the necessity of speculation and conjecture. 



The relation, however, which the slight modification of the terminal caudal 

 vertebrae above alluded to bears to the presence of a large, horizontal, liga- 

 mentous fin, induced me some time ago to examine such specimens of skele- 

 tons of Ichthyosauri and Plesiosauri, having the tail complete, as were access- 

 ible in London, with the view to obtain evidence of the possible existence or 

 trace of a similar structure in these species ; but I was then unable to obtain 

 from the imbedded vertebrae satisfactory proof of their exchanging a com- 

 pressed for a depressed form at this part, or deviating in any way (save in 

 size, number of articular surfaces, and, in the Plesiosauri, in the greater ex- 

 cavation of their anterior and posterior surfaces of articulation) from the rest 

 of the vertebrae of the tail. I concluded, therefore, that these air-breathing 

 inhabitants of the ancient deep, partly from being cold-blooded, as the condi- 

 tion of the ossification of their skeleton proves, and therefore slow breathers, 

 and more especially on account of the superaddition of posterior paddles, were 

 devoid of any locomotive organ analogous to the tail-fin of the Cetacea; and 



