268 APPE^^DI 



ble those of the horse very strongly, the cro-»vns of the incisors shovring 

 the peculiar invagination s^een in the incisors of the horse. 



7.— Left Jower molar oi Hippoi.herivni gracile.. three-fonrths natural size 

 (Gauciry). The lobe aV is now completely separated and 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 tlie valleys by a deposit of cementum 

 and the consequent attrition in mastication produce a marked change iu 

 appearance from that seen in Anchitherium, but by close ob:.er\ation the 

 strictest homology is seen to exist. 



8.— Left upper molar of a species of Eqnvs (modem horse) natural size. 

 The internal lobes ai iii are connected with the cross ridges ace, JKC. The 

 only difference of generic value between Equm and Ilippiclium (a near rela- 

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

 ternal lobes ai.pi ; in Equus ai is greatly enlarged and somewhat flattened; 

 iu Hippidium the lobes are almost equal. 



What has caused these chanpres ? 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 

 ^vhich 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 

 bnnodont 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 enatnel 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 areater and greater mobility of the mandibttlar 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 

 broadenincT 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 ctisps and 

 ridges, thereby ])roducing 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. 



