512 R. Owen, Esq., on the 



that the hinder paddles principally served the purpose, in the absence of such 

 a horizontal terminal fin, in bringing the head to the surface for the purpose 

 of breathing. 



Having recently examined many saurian skeletons now in London, the 

 greater part of which have been disencumbered of their earthy shroud by the 

 chisel of Mr. Hawkins, a condition of the tail which, on a former occasion, in 

 a single instance had arrested my attention, but without calling up any theory 

 to account for it, now more forcibly engaged my thoughts, from observing 

 that it was repeated, with scarcely any variation, in five instances. The con- 

 dition to which I allude is an abrupt bend or dislocation of the tail at about 

 one-third of its whole length distant from the end; generally about the thir- 

 tieth caudal vertebra in the Ichthyosaurus communis ; the terminal por- 

 tion continuing, after the bend, almost as straight as the portion of the tail 

 preceding it. In short, the appearance presented is precisely that of a stick 

 which has been broken, and with the broken end still left attached, and de- 

 pending at an open angle. 



Now there is no modification of the vertebras, where this bend takes place 

 in the tail of the Ichthyosauri indicative of the tail having possessed, during 

 the lifetime of the animal, greater mobility at this particular point than at any 

 other; and it is scarcely possible to conceive how any force operating on the 

 dead carcase at the bottom of the sea, or when it was imbedded in sediment- 

 ary deposits, could produce a fracture or dislocation of the tail at the precise 

 point at which we find it in so many specimens, otherwise very dissimilar in 

 regard to their general position and degree of perfection. I incline to believe, 

 therefore, that the appearance is due to the operation of some force acting 

 upon a ligamentous fin attached to that part of the extremity of the tail which 

 is thus bent down ; and that this force operated while the dead animal was 

 floating and buoyed up by the gases generated by putrefaction, and when 

 the ligaments and other connexions of the caudal vertebrae had been so far 

 loosened as to allow the force which previously would have bent down the tail 

 in a general curve like the top of a fishing-rod, to overcome the diminished 

 resistance of the decomposing joints immediately proximal of its point of 

 action, and dislocate the vertebrae at that part. The two portions of the tail, 

 like the rest of the skeleton, would then continue to be held together by the 

 common integument, until the rupture of the parietes of the abdomen would 

 allow the pent-up gas to escape, and the body to sink to the bottom. 



In thus descending through a sufficiently tranquil medium, the back being 

 weighted with the heavy vertebral column, would be most likely the first part 

 to touch the bottom ; and if this consisted of soft clay or mud, a heavy trunk. 



