456 



MAMMALIAN ANATOMY 



FIG. 383. 



antero-posterior diameter of the tooth and a special increase in the 

 size of the basal cusps. 



From this last stage of the variation of the second premolar to the 

 form of the sectorial tooth (Fig. 382) is a short step. I think we are 

 justified in supposing the further process of evolution to have been as 

 follows : continued use has increased the size of the deuterocone, and 



pressure (Fig. 383) has moved it forward 

 until it is nearly opposite the anterior 

 basal cusp, or protostyle ; the posterior 

 basal cusp, or tritocone, has become longer, 

 and with the principal cusp, or protocone, 

 forms the cutting blade, which is now 

 supported almost entirely by the large 

 posterior fang. 



The evolution of the lower molar from 

 the premolar form would be, presumably, 

 a comparatively simple process, requiring 

 merely the lengthening of the posterior 

 basal cusp. We have, however, no evi- 

 dence from variations which points out this 

 as the method ; on the contrary, we have 

 some evidence which is in favor of the view 

 that the lower sectorial is derived from the 

 tuberculo-sectorial molar. This is pre- 

 sented by the occurrence in one specimen 

 of this tooth of an anomalous small poste- 

 rior outer fang, which was probably croAvned 

 by a small cusp. This structure was doubtless a part of a rudimentary 

 heel, and more especially the outer part bearing the hypoconid. It is 

 not an unusual anomaly in the lower deciduous sectorials (Fig. 364) 

 where the talon and metaconid are well developed, the latter sometimes 

 being placed on the inner side of the crown. 



Inferences drawn from variations in the deciduous dentition must 

 be received with caution ; we do not yet know the relation existing 

 between the deciduous and permanent dentitions, or even which is 

 primary and which is secondary. 



This third cusp on the lower molar might be regarded as the rudi- 

 mentary protoconid of the original tritubercular stage, the shearing 



DIAGRAM OF APPOSITION OF UPPER 

 AND LOWER POSTERIOR TEETH 

 WHEN UPPER SECOND PREMOLAR 

 HAS AN INNER CUSP. 



Heavy lines represent lower teeth ; light 

 lines, upper teeth: a, anterior cusp; 

 6, middle cusp ; c, posterior cusp ; d, 

 talon ; e, inner cusp. 



