KINETOGENESIS. 321 



where the external face of the lower jaw crosses the 

 upper jaw from within outwards (ental); while in other 

 types the food is masticated on the side where the 

 lower jaw passes the external edge of the upper jaw 

 from without inwards (ectal). While masticating with 

 one side of the jaws, the opposing dental series of the 

 other side are not in contact. All mutual effect of the 

 teeth of one jaw on the other could therefore appear 

 on the side temporarily used for mastication only. 

 Among recent Un- 3 a i' 



gulata the rumi- '"^^ ^ 



nants present the {/^^ ^X^ ^^^OlIu^^^^'* 

 ental mastication ; ^sM''i^.S^.~'"'.'.'a"'^ 



the rhinoceros and " <*' 



horses, the ectal ; c 



and rodents, the 



proal. Ryder is of 



the opinion that the n]' 



mastication of the \.>^(|i iijwv ir. \f\r.v 



TD 1 -v • r M^lil in 1 im \( V 



Proboscidia IS pall- « ■•- MiAii>ii Riui/ J 1 



nal, but i have not Fig. 89.— Cervus, molars: <i, superior, exter- 



been able to satisfy "^' ^^^™ ■ '''• '^''- inferior view; c, inferior molars, 



superior view ; from Ryder. 



myself of this. 



When the crests of the inferior molars were devel- 

 oped, their relation to the crests of the superior molars 

 was always anterior in mastication. That is, the in- 

 ferior crest, in the closing of the jaw, collides with the 

 crest of the upper molar, with its posterior edge against 

 the anterior edge of the latter. This is because : firs/, 

 as to position, the two anterior cusps of the lower 

 molar are the remains of the anterior triangle which 

 fit originally between two superior molars, and be- 

 cause, in the closing of the jaw, these cusps continue 

 to hold that position ; and second, as to function, be- 



