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forms, in that class, so important an organ of locomotion ; and the 

 improbability that its presence would have been suspected, had the 

 cetacea been known only by their fossil remains, in consequence of 

 the fin having consisted entirely of decomposable and unossified 

 material. 



He states, that the flattened shape of the terminal vertebrae, which 

 gives the only indication of the horizontal fi.n — and which cha- 

 racter is not present in all the cetacea — is not recognisable in the 

 skeletons of the Ichthyosauri and Plesiosauri ; but he proceeds to 

 describe a condition of the tail in the skeletons of the Ichthyosauri, 

 which, he conceives, affords an indication of a structure in the ex- 

 tinct animal, analogous to the tegumentary fin of the cetacea, and 

 wliich has not been suspected by the authors of the conjecturally- 

 restored figures of the Ichthyosauri, already published. The condi- 

 tion alluded to, is described as an abrupt bend of the tail about 

 one-third of its whole length distant from the end ; and at the thir- 

 tieth caudal vertebra in the Ichthyosaurus communis; the broken 

 portion continuing, beyond the dislocation, as straight as in the part 

 which precedes it. As there is no appearance of a modification of 

 structure in the dislocated vertebrae, indicative of the tail having 

 possessed more mobility at that point than at any other; and as the 

 dislocation has taken place at the same point in seven specimens ex- 

 amined by the author, he conceives that it must be due to some 

 cause operating in a pecxiliar manner on the dead carcase of the 

 Ichthyosaurus, in consequence of some pecuHarity of external form, 

 while it floated on the surface of the sea. 



A broad tegumentary fin, composed of dense but decomposable 

 material, might have been attached to the terminal portion of the 

 tail ; and such a fin, either by its weight, or by presenting an ex- 

 tended surface to the beating of the waves, or by attracting preda- 

 tory animals of strength sufiicient to tug at, without tearing it off^, 

 would occasion, when decomposition of the connecting ligaments 

 had sufficiently far advanced, a dislocation of the vertebrae imme- 

 diately proximate of its point of attachment. The two portions of 

 the tail, with the rest of the skeleton, would continue to be held to- 

 gether by the dense exterior integument, until the rupture of the 

 parietes of the abdomen, at some yielding point, had set free the gases 

 generated by putrefaction ; and the skeleton, having undergone cer- 

 tain partial dislocations, from the decomposition of the more yield- 

 ing ligaments, would subside to the bottom, and become imbedded 

 in the sedimentary deposits, exhibiting the fracture of the tail al- 

 luded to. 



With respect to the relative position of this conjectured, caudal, 

 tegumentary fin of the Ichthyosaurus, Mr. Owen cannot perceive 

 any indication of its horizontality in the forms of the vertebrae, 

 which he supposes to have supported it ; and he regards the super- 

 addition of posterior paddles in these air-breathing marine animals, 

 as a compensation for the absence of that form of fin, which is so 

 essential in the cetacea, for the purpose of bringing the head to the 

 surface of the sea to inhale the air. On the other hand, a vertical 



