Dislocation of the Tail of the Ichthyosauri. 5l3 



like that of the Ichthyosaurus, would immediately subside therein back down- 

 wards, while the broken tail, supported by its flat surface, would afterwards 

 be dragged down at an angle with the trunk, and with its extremity directed 

 towards the ventral — now, as when the body floated, the uppermost side of the 

 body : this is, in fact, the position in which the broken part of the tail is ge- 

 nerally placed in the fossil skeletons. 



The prevailing amount of dislocation which most of the skeletons of the Ich- 

 thyosauri present, is exactly such as might be conceived to take place in a dead 

 animal body buoyed up sufficiently long to allow of a partial separation of the 

 connecting ligaments and cartilages ; the probable toughness of the integu- 

 ments of the Enaliosaurians would tend to retain the gases generated by pu- 

 trefaction sufficiently long to enable this extent of decomposition to take place 

 while the animal floated, at least in regard to the ribs and bones of the pectoral 

 and pelvic arches. The bones of the paddles on the contrary, and those of the 

 tail, being connected together by denser ligaments and surrounded by a less 

 proportion of soft animal matter, would be more likely to sink comparatively 

 undisturbed, with the rest of the trunk, when the buoyant gases were ulti- 

 mately set free. 



The dead carcasses of the Ichthyosauri would of course be subject to such 

 dislocations and separations as the attacks of predatory animals on a floating 

 dead body would produce ; but the appearance in the tail of the Ichthyosaurus 

 here alluded to, is too uniform and common to be due entirely to an accidental 

 and extrinsic cause. I am therefore disposed to attribute it to an influence 

 connected with some structure of the recent animal ; and most probably to 

 the presence of a terminal tegumentary and ligamentous caudal fin, which, 

 either by its weight, or by the force of the waves beating upon its extended 

 surface, or by the action of predatory animals of strength sufficient to tug at 

 without tearing it off", might, under the circumstances already mentioned, give 

 rise to a dislocation of the caudal vertebrae immediately proximal of its at- 

 tachment. We have evidence, however, in the form of the vertebrae that the 

 supposed fin was not a horizontal one, and I have already observed that the 

 superaddition of posterior paddles in these air-breathing marine animals is the 

 compensation for the absence of such an organ ; and, therefore, a tegument- 

 ary fin, if developed at the extremity of the tail, would probably be placed in 

 the position best adapted to produce with rapidity those lateral movements of 

 the head most needed in a short-necked predatory aquatic animal. 



Since the preceding notes were written and read to the Society, I have had 

 the opportunity of detaching some of the terminal caudal vertebrae of an 

 Ichthyosaurus communis from the substance in which they were imbedded ; 



