8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 72 



washed away from between the teeth. The dentition is therefore 

 chiefly used for grasping the food and holding it fast. In the most 

 primitive whales the mouth did service as an implement for catching 

 fish. The jaws were used in exactly the same manner as in the shell- 

 drakes, Mergns, and they were produced forward as a long slim beak, 

 a kind of tweezers, influenced not only by the use to which they were 

 put, but also by the pressure of the water during swimming forward. 

 The HycBnodon-Wy^e. dentition which the most primitive whales 

 inherited, with teeth of considerable size, diversified form, and of 

 typical number, at first becomes more simple. The upper molariform 

 teeth lose the inner cusp and the inner root, and the crown under- 

 goes compression. A further step in the reduction is that the crowns 

 of the cheekteeth, or at least of most of them, acquire a serrated 

 anterior and posterior margin. Next the two remaining roots, 

 foremost and hindmost, of the cheekteeth fuse into one, and the 

 serrations of the crowns are reduced and obliterated. The size 

 at the same time is reduced, and the form becomes simply conical so 

 as to resemble that of the incisors and canine, which in their turn 

 undergo reduction. While this is happening the number of teeth in 

 the long jaws is increased, no doubt because in the place of the few 

 quite large teeth there spring up many smaller ones ; scarcely by the 

 actual splitting up of the few. Perhaps also in the beginning some 

 of the milk teeth came to take a place in the series with the permanent 

 ones, without, however, the entire milk dentition's intercalation in the 

 permanent set. The number of teeth grows greater and greater, far 

 beyond the typical, while the individual teeth become smaller and 

 smaller. Those at the front and back of the series become especially 

 stunted, frequently disappearing from the intermaxillary. The 

 enamel covering of the teeth becomes thin or disappears entirely. 

 What later happens to the dentition depends on the use to which it is 

 put. It may happen that there comes to be no use whatever for it, 

 and that it consequently disappears. Or it may, wholly or in part, be 

 once more put to heavy use and be modified to this end ; or a single 

 tooth may take on power while all the others atrophy. 



The succession of teeth, which in the most primitive whales took 

 place in the ordinary way, ceases. It is not clear how this happens. 

 Judging from investigations of the teeth in embryos of the higher 

 cetaceans it might appear, at least sometimes, as if it were retained 

 milk teeth that are found in the adult animal's dentition — as if the 

 successors to the milk teeth had disappeared. Such, however, is 

 scarcely the explanation. Most probably it is really the actual perma- 



