268 



ble those of the horse very strongly, the crowns of the incisors showing 

 the peculiar invagination seen in the incisors of the horse. 



7. Left lower molar of Hippolherium gracile, three-fourths natural size 

 (Gaudry). The lobe ai' is now completely separated aud the ridge k rises 

 to a level with the other cusps. The heel h is also elevated and connected 

 by a strong ridge. The filling up of the valleys by a deposit of cementum 

 and the consequent attrition in mastication produce a marked change in 

 appearance from that seen in Anchitherium, but by close observation the 

 strictest homology is seen to exist. 



8. Left upper molar of a species of Equvs (modern horse) natural size. 

 The internal lobes at, pi are connected with the cross ridges ace, pec. The 

 only difference of generic value between Equus and Hipjndivm (a near rela- 

 tive of the horse) is seen in the relative size of the antero and postero-in- 

 temal lobes ai,pi ; in Equus ai is greatly enlarged and somewhat flattened; 

 in Hippidium the lobes are almost equal. 



What has caused these changes ? In regard to tooth struc- 

 ture generally, Mr. J. A. Ryder has given us a most excellent 

 treatise "On the Mechanical Genesis of Tooth Forms,"* in 

 which he shows that the jaw movements of animals are 

 intimately related to the modification of the component lobes, 

 crests, and ridges of the crowns of the molar teeth. He also 

 points out that the restricted jaw movements, in which the 

 mouth is simply opened and closed, are associated with the 

 bunodoat molar ; that the various kinds of excursive mandib- 

 ular movements have been developed progressively ; "that as 

 these movements have increased in complexity there has been 

 increase in the complexity of the enamel foldings." 



If we attempt to apply these facts to the ancestry of the horse, 

 it is by no means difficult to perceive that gradual change of 

 habitat, causing a corresponding change in diet, would also 

 compel greater and greater mobility of the mandibular articula- 

 tion for proper trituration of the new food. The movements of 

 the lower jaw in these animals have assumed a lateral direction, 

 which affords, as I believe, a sufficient explanation for the 

 broadening of the crowns and the lateral flattening of the cusps. 

 The obvious effect of force continually applied in this direction 

 would be to wrinkle the enamel covering of the cusps and 

 ridges, thereby producing the accessory pillars seen in the 

 higher types. By this method, I believe, a more and more 

 complex grinding surface has been produced. 



* Proceedings Academy Natural Sciences, Philadelphia, 1878. 



