ICINETOGENESIS. 319 



guished in sparoid fishes, and in placodont and dia- 

 dectid reptiles. Canine-like teeth, or pseudo-canines, 

 appear in clepsydropid and crocodilian reptiles, and 

 in saurodont fishes. Canine-like incisors appear in 

 the Clepsydropidae. The variety of character in these 

 structures presented by the Mammalia to be consid- 

 ered is great, and the principles deduced from obser- 

 vation of them are applicable to the Vertebrata in 

 general. 



As mechanical causes of the origin of dental modi- 

 fications, I have enumerated the following : 



1. Increase of size of a tooth, or a part of a tooth, 

 is due to increased use, within a certain maximum of 

 capacity for increased nutrition. 



2. The change of direction and use of a tooth take 

 place away from the direction of greatest, and in the 

 direction of least resistance. 



3. It follows, from their greater flexibility, that 

 crests of crowns of teeth yield to strains more readily 

 than do the cusps. 



4. The increase in the length of crests and cusps 

 in all directions, and therefore the plications of the 

 same, is directly as the irritation from use to which 

 their apices and edges are subjected, to the limit set 

 by the destructive effects of such 'use, or by the re- 

 cuperative energy of nutrition. 



5. The direction of growth of the branches of a V, 

 or of the horns of a crescent, will be the direction of 

 movement of the corresponding parts of the opposite 

 jaw. 



Before giving a review of the various dental types 

 of Mammalia, I wish to describe some special exam- 

 ples where the effect of mechanical causes is most ob- 

 vious. I therefore first repeat the observation of Ryder 



