a. _Tail of Spinax Acanthias._ b. _Tail of Ichthyosaurus Tenuirostris_, (Buckland.)] In illustration of this view, I would fain recommend to the reader a simple experiment. Let him procure the tail of a common dog-fish, (fig. 48, _a_,) and cutting it across about half an inch above where the caudal fin begins, let him boil it smartly for about half an hour. He will first see it swell and then burst, all around those thinner parts of the fin that are traversed by the caudal rays,—wholly mucoidal, as shown by this test, in their texture, and which yield to the boiling water, as if formed of isinglass. They finally dissolve, and drop away, with the surrounding cuticular integument; and then there only remains, as the insoluble framework of the whole, the bodies of the vertebræ, with their neural and hœmal processes. The tail has now lost much of its ichthyic character, and has acquired, instead, a considerable degree of resemblance to the reptilian tail, as exemplified in the saurians. I have introduced into the wood-cut, for the purpose of comparison, the tail of the ichthyosaurus, (_b_.) It consists, like the other, of a series of gradually diminishing vertebræ, and must have also supported, says Professor Owen, a propelling fin, placed vertically, as in the shark, which, however, from its perishable nature, has in every instance disappeared in the earth, as that of the dog-fish disappears in the boiling water. It will be seen that its processes are comparatively smaller than those of the fish, and that the bodies of its vertebræ are shorter and bulkier; but there is at least a general correspondence of the parts; and were the tail of the crocodile, of which the vertebral bodies are slender and the processes large, to be substituted for that of the enaliosaur here, the correspondence would be more marked still. After thus _developing_ the tail of the reptile out of that of the fish,—as the cauldron-bearing Irish magician of the tale developed young ladies out of old women,—simply by _boiling_, let the reader proceed to a second stage of the experiment, and see whether he may not be able still further to develope the reptilian tail so obtained, into that of the mammal, by _burning_. Let him spread it out on a piece of iron hoop, and thrust it into the fire; and then, after exposure for some time to a red heat has consumed and dissipated its merely cartilaginous portions, such as the neural and hœmal processes, with the little pieces which form the sides of the neural arch, and left only the whitened bodies of the vertebræ, let him say whether the bony portion which remains does not present a more exact resemblance to the mammiferous tail—that of the dog, for example—than any thing else he ever saw. The Lamarckians may well deem it an unlucky circumstance, that one special portion of their theory should demand the depreciation of the heterocercal tail, seeing that it might be