XXXVI.—Note on the Dislocation of the Tail at a certain point observable in the Skeleton of many Ichthyosauri.

Abstract

Plate XLII. 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 existence, 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 vertebræ above alluded to bears to the presence of a large, horizontal, ligamentous fin, induced me some time ago to examine such specimens of skeletons of Ichthyosauri and Plesiosauri, having the tail complete, as were accessible 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 vertebræ satisfactory proof of their exchanging a compressed 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 excavation