The first great step in the anatomy of the Cetacea is unquestionably due to Cuvier; but his dissections were almost confined to the genus _Delphinus_, or the common Porpoise of our coasts. I repeated all his dissections, and found them, as they almost always were, scrupulously exact; but when I came to examine Cetacea with whalebone instead of teeth, I was surprised to find how different, in fact, the anatomy of the two great families was. Scarcely in any great natural family do we find Cuvier's favourite theory of anatomical and physiological co-relations so entirely at fault as in the Cetacea. The teeth or whalebone, as natural-history characters, lead to no results; the whole structure of the interior defies all _a-priori_ reasoning. The brain in whalebone-whales does not fill the interior of the cranium; so that the capacity of the one is no measure of the solid bulk of the other. Their food is various, having no relation to the teeth or buccal appendages; vascular structures surround the spinal marrow, and extend in the _Balaenopterae_ into the cavity of the cranium, which seem to be without any analogy in other mammals, or, at the least, a very obscure one, and whose functions are wholly unknown. Cetacea might with some propriety be divided into whales with whalebone, and whales with teeth. Those with whalebone have rudimentary teeth in both jaws in the foetal state. Fossil Cetacea exist, and they seem to have been of both kinds, but, no doubt, were generically and specifically distinct from the recent. Judging from the remains of those I have seen, I am inclined to think that those with teeth were of a stronger and firmer build in the skeleton than those called recent; that the neck was longer, and the caudal portion of the column shorter than in the recent kinds, and that they approached the Saurians in form. There is a remarkable want of symmetry in the crania of some of the Cetacea; but most remarkable is the cranium of the Narwhal. Of this fact I have already spoken, in the article published in the Transactions of the Royal Society of Edinburgh. _Delphinus Phocaena. Dissection of a small Cetacean sent to me from Orkney in the month of May 1835._--This species is said to abound on the coasts, and to furnish a kind of fishery to the inhabitants. On dissection we found 81 vertebrae, exclusive of the cephalic. The species must be quite distinct from those previously and subsequently examined by myself and many others, in which the number of vertebrae ranged from 61 to 66. It is also, I think, distinct from the specimen I saw in Dr. R. Hunter's Museum in Glasgow, in which the number of vertebrae was 90, exclusive of the cephalic in all the cases. Thus it stands with regard to the Cetacea called Porpoises and Dolphins. In certain species of _Delphinus_ the vertical column is composed of 61 vertebrae, in others of 65, in others of 66, in others of 81, in others of 90.