Whalebone Whales never possess permanent teeth as well as the baleen; but in the foetus are more than traces of true teeth, which, however, never arrive at maturity. The whalebone itself is described later (p. 354). That the Whalebone Whales possess teeth while in the foetal condition was discovered so long ago as 1807. It has since been confirmed by many observers. Not only is there one set of teeth developed in the foetal _Balaenoptera_ but two, of which one comes to a greater maturity; the other, in fact, remaining at a very early stage of development. The more complete dentition belongs to the milk series, as is the case with the Toothed Whales. A very interesting conclusion with regard to the derivation of the simple conical teeth of Whales seems to follow from the development of these structures in Balaenoptera. There are in the young foetus fewer teeth than in the more advanced embryo. Now in the younger embryo some of the teeth are furnished with more than one cusp; they are bi- or even tri-conodont. As Sir R. Owen observed, the teeth--some of {347} them--are literally double teeth. This is a suggestion of the more complicated teeth of the Zeuglodonts, and shows so far that the simple conical teeth of existing Whales (cf. however the Platanistidae) are not by any manner of means so primitive as their actual structure would undoubtedly lead one to believe. Further than this, the greater number of teeth in the older embryo coincided with the disappearance of these double teeth, which seem to split up into the simple conical teeth. The Toothed Whales are not furnished with baleen, but with teeth only. These teeth are more or less numerous, their arrangement being of value in the classification of the group; a matter which is dealt with later. In the Narwhal, whose dentition in the adult is reduced to the well-known tusk or tusks (properly developed only in the male), there is a complete foetal dentition. A very curious fact has been elucidated by Professor Kuekenthal about the dentition of the Common Porpoise. It appears that in this Cetacean the two teeth corresponding to each other of the two dentitions may fuse into a single tooth, which has in consequence a double crown. It may be that this is the case with the Platanistid _Inia_, and that its diconodont teeth are not, therefore, a reminiscence of the comparatively complicated teeth of the ancient Zeuglodonts. The internal organs of Whales which show the greatest peculiarities as compared with other mammals are the stomach, the lungs, and the diaphragm. Whales always possess a complicated stomach divided into many, but into a variable number of, chambers: there are as few as four in some, as many as fourteen in Ziphioids.