54 EVOLUTION OF MAMMALIAN MOLAR TEETH 



A beautiful illustration of the fundamental pattern of trigon and talon 

 in the upper human molars is shown in Hose's figure 4 of the molar of a 

 six months' child (Fig. 42). The protocone makes the apex, and is con- 

 nected by two spurs with the two external cusps, the space between 

 which is comparatively open as in the primitive forms. 



Thus the homology of the antero-internal cusp of the upper molar 

 with the protocone is well supported by palaeontology and by dental 

 mechanics, but how shall we meet the embryological counter-evidence 

 established by the agreement between the independent investigations of 

 Rose and Taeker ? 



This is also, I believe, explained by a study of the fossil forms. As 

 we have seen in the most primitive types the protocone was the most 

 prominent cusp in both jaws, but in course of later development of the 

 upper molars, during the Cretaceous and Eocene periods, the protocone was 

 depressed to the level of the paracone and metacone (see the primitive 

 Carnivora, Creodonta, and Insectivora). On the other hand, in the lower 

 molars the protoconid retained its relatively prominent position and size. 

 If the ontogenetic development of the lower molars corresponds with the 

 ancestral order, it is probably because the relative primitive position of 

 the cusps was conserved ; whereas in the upper molars, in which there is 

 less correspondence, it was lost. I find in the lower Eocene Ungulates 

 that the paracone and metacone are more important cusps than the 

 protocone. So far as the fossil Primates of the lower Eocene are known, 

 we find the protoconid is the most prominent cusp in the lower molars, 

 while in the upper molars the protocone is less prominent than the 

 external cusps. Rose's argument really turns therefore upon the expecta- 

 tion that foetal development should repeat ancestral history of the 

 Cretaceous period ! As the flattened form of the crown is from the start 

 a Ca?nozoic type, we should hardly expect the order of cusp succession to 

 invariably revert to a Mesozoic type. While not thoroughly convincing, 

 there is a great deal of force in this way of meeting the embryological 

 data. 



Nomenclature. Rose (p. 400) apparently mistakes the homologies of 

 the lower molar cusps of man, for he has overlooked the fact that the 

 primitive anterior lingual cusp, or paraconid, has degenerated in the 

 Primates (excepting in a few Lemurs) while it persists in Diddphys, It 

 is not seen in the human lower molar at all. Its declining stages mark 

 the loss of sectorial function and can be readily followed in the lemurs, 

 and fossil monkeys ; as shown by Cope and myself it degenerates while 

 the hypocone in the upper molars develops. It follows that the anterior 

 lingual in man is the metaconid, while in Didelpliys it is the paraconid 

 and the mid-lingual is the metaconid. The posterior lateral cusp is 

 undoubtedly the hypoconid. Rose proposes the term " pentaconid " for 



