34 EVOLUTION OF MAMMALIAN MOLAR TEETH 



confirms the second of the above alternatives, viz., that this heel is to 

 l>e compared to the hypocone of the tubercular-sectorial crown. Further 

 confirmation is seen in the fact that this heel is not above the level 

 of the internal cingulum, as in the metacone of all the trieonodonts, 

 but is continuous with the broad shelf-like projection of the internal 

 cingulum, which is well represented in the internal aspect of the 

 Dipliii-i/inxlnii (Fig. 20) molars. The concave internal slope of the 

 protocone descends into this shelf and the cingulum rises at the margin 

 into numerous crenations, which cannot properly be called cusps. The 

 Diplocynodon (Fig. 20) molar presents a decided advance upon that 

 of Emieoilon in the development of the paracone, which is much more 

 prominent. In Amphitherium (Figs. 4, 15, 17), the paracone is 

 subequal to the protocone in several of the molars, and the heel is 

 on the level of the internal cingulum, from which, according to Owen, 

 there arise one or two small cusps. 1 Internal cusps which develop in 

 this manner are from the first separated from the external cusps by a 

 longitudinal valley instead of being united with it by divergent ridges, 

 and cannot therefore at any stage possess a sectorial blade, such as 

 is more or less distinctly developed in the Spalacotherium and Stt///t- 

 codon molar. 



5. It follows also that the triangle of cusps presented by the 

 Pcraspalax molar (Fig. 22) cannot, with probability, be considered as 

 representing a tritubercular stage, and that the Amphitheriidse furnish 

 the key to the mode of derivation of the internal cusps of the 

 molars of the Peralestida? (Fig. 12). The inferior molars of Pcraspnl.' 

 and Pan ril on (Fig. 26) are apparently very similar, consisting 

 of a prominent external cone, and two internal cusps followed by a 

 third cusp at the end of the crown. As pointed out in the synopsis of 

 molar types, this internal surface strongly suggests the Dryolestcs pattern 

 (Figs. 31-34), but may be clearly distinguished by the absence of trans- 

 verse ridges and the presence of a longitudinal valley between the 

 cusps instead of a transverse valley opening inwards. The internal 

 cusps have probably, therefore, arisen from the internal cingulum,' 2 

 but these molars do not seem to be a later development of the 

 Amphitherium type (Fig. 1-5), because both the paracone and meta- 

 cone are wanting, the main cone showing no trace of the lateral cusps 



1 As previously stated the writer has not personally examined the internal surfaces 

 of the molars of this genus. 



2 Numerous instances of the origin of molar cusps from the cingulum might be 

 cited. One of the most important is seen in the transition from a tritubercular to a 

 quadritubercular superior molar by the addition of the postero-internal cusp which is 

 primitively a cingule ; this was first demonstrated by Dr. Harrison Allen, op. cit. 

 Mivart (Jour, of Anat. and Pht/fs., Vol. II., p. 138) shows how the four cusps of the 

 Insectivore molars are frequently fortified by additional cusps from the cingulum. 



