TOOTH-GENESIS IN THE CANID^-, 477 



cone, the next form would be that of a tooth with a main central 

 cone and minute lateral cusps. Such teeth are to be found 

 in Micoconodon. The prominence of the Internal Cingulura 

 with the formation of a central cusp upon it is clearly to be 

 seen in such teeth as those of Am'pMtlierium Prevostii and 

 Feraspalax ; whilst the presence of the Secondary cone is to be 

 noted in the molar teeth of almost all forms. Erom the con- 

 sideration of such forms as these, I think it is evident that 

 Palaeontology furnishes quite as strong evidence in favour of 

 such a theory as I have attempted to describe, as it does for 

 either the IVitubercular or Multitubercular theory. 



(ii.) Emhryological evidence. — The strongest confirmation of 

 this view is to be found in the fact that in all the orders in 

 which the cusp evolution has at present been worked out, the 

 Primary cone (Paracone and Protoconid) develops first in every 

 instance, in both upper and lower jaws. 



That the Cingulum is a structure of great antiquity and im- 

 portance is borne out by the fact that it appears developmentally 

 very shortly after the Primary cone can be distinguished. In 

 the teeth of the fcBtal Dog it is comparatively very largely 

 developed, especially in the incisors. The internal cingulura is 

 more marked in the lower members of both the Thooid and Alo- 

 pecoid series than it is in the higher ; and its presence, together 

 with the small anterior and posterior cingulum-cusps to which it 

 gives rise, is to be noted in all the teeth of the Dog, both milk 

 and permanent. So far this theory is in accordance with all 

 known embryological facts : the diflSculty arises in connection 

 with the secondary cusps. To explain my meaning more fully : 

 if the development of the cusps of dpm.* be traced, the so-called 

 Protocone of that tooth commences to appear before the Meta- 

 cone, though the latter soon surpasses it in size ; this result 

 agrees with those of Eose for the Primates and Marsupials 

 (the teeth in the latter I have given reasons for regarding as 

 milk-teeth) ; whereas in the development of ^ of the Dog the 

 Metacone develops before the Protocone of that tooth, in 

 accordance with Taeker's results in the Ungulates. 



A further difficulty is to be found in connection with the 

 Metacone of the upper carnassial teeth of the Carnivora and 

 with the Hypoconid of the lower. We have seen that two 

 factors enter into the formation of both these cusps, namely, the 

 Posterior Cingulum-cusp and the Secondary Coue, but they do 



LINN. JOUBN. — ZOOLOGY, VOL. XXV. 39 



