TOOTH-aENESIS IN THE CANID^. 453 



that the upper permanent carnassial is preceded by a deciduous 

 tooth molariform in character, and that the penultimate de- 

 ciduous premolar has the general characters of the permanent 

 carnassial. The same holds in the lower jaw in relation to pm.^ 

 and m7. 



Again, if the upper milk and permanent carnassials be com- 

 pared, it will be seen that in the latter three external cusps are 

 present, the posterior being the cingulura-cusp, whereas in the 

 former the division of the Metacone into two cusps is not 

 so clearly distinguishable. In the second functional deciduous 

 ])remolar ( dpm.- ) there is but the very faintest indication of a 

 second cusp externally, which is very much more marked in its 

 permanent successor. The same thing is to be noted in the 

 lower jaw, but in a lesser degree. From these considerations it 

 will be seen that the teeth of the permanent dentition show au 

 increase both in the number and size of the cusps over the corre- 

 sponding milk-teeth ; in other words, the teeth of the deciduous 

 are simpler than those of the permanent dentition. This fact is 

 still more strikingly shown if the biting-surface of the crowns of 

 the teeth be examined. In fii;. 3 is shown the bicinsr-surface 



A, the biting-surface of First Permanent Molar, and B, of the Fourth 

 Deciduous Premolar of the Dog. 



of dprru* and T^ of Canis familiaris ; in the former there are 

 indications of four cusps, whereas in the latter seven are to be 

 seen. 



But this comparison brings out another very important fact. 

 I think it will be generally admitted that the cusp {pr) in ^ is 

 the Protocone ; and on comparison w-ith dpm.'* it will be seen that 

 iu the latter this cusp, the Protocone, is entirely absent. 



This conclusion I think is very damaging to the Tritubercular 

 theory as I understand it. 



The difficulty in homologiz"ng the Protocone in the various 

 teeth of Canis familiaris has already been pointed out, but it is 



LINN. JOITiJN. — ZOOLOGY, VOL. XiV. 37 



