ORDINAL TYPKS OF MOLARS: RODENTIA 



149 



more or less evident traces of the primitive pattern. The milk 

 teeth also still retain vestiges of a triangular pattern with three 

 main tubercles, internal to which is a deep notch or internal folding. 

 This notch in the adult teeth extends entirely across the crown, 

 forming the double transverse ridge which so deeply divides the 

 crown that the anterior and posterior moieties have been supposed 

 by Marett Tims l to represent the fusion of two originally separate 

 elements. This notch, however, is entirely secondary. 



The ancient pattern of the molars is bomologized by Forsyth 

 Major 2 himself with that of the molars of Pdycodus, a primitive and 

 strictly tritubercular primate ; it certainly suggests as the ancestral 

 condition a triangular to quadrate, low-cusped, brachyodont, three- 



7376 



D 



pr. 



FIG. 114. Apparent traces of trituberculy in Lagomorpha (Duplicidentata). After Forsyth Major. 



A. Upper molar of a primitive Oehotonid (Lagomyid) Tltanoiwis fontanitcsi from the Middle 

 Miocene of Em-ope. Note : (1) the beginning of the groove on the internal side, which in the 

 later Hares, etc., sinks inwards and divides the crown into two portions ; (2) apparent vestiges 

 of the tritubercular pattern. 



B. Anterior view of the same tooth. Note the reduction of the external roots, the hyper- 

 trophy of the internal root, the spreading of the enamel upon the anterior and inferior portions 

 of the crown. 



C. Lcpus cuniculus, milk molar 1, showing retention of much less specialized condition than 

 in the adult. 



D. Second upper molar of a Flying Squirrel (Ptertmvys i,l<iaotis) to illustrate the ancestral 

 type of molars from which the specialized type in the Duplicidentata molars proabably arose. 

 (Cf. Fig. 115.) All figures .. 



rooted tooth, with the protocone centrally placed. This type of tooth 

 is associated with omnivorous or insectivorous habits and a chiefly 

 vertical motion of the mandible, as compared with the oblique to 

 fore-and-aft grinding motion and herbivorous diet to which the adult 

 teeth of the Duplicidentata are adapted. Thus the milk teeth of even 

 this highly specialized group of Rodents revert to a tritubercular 

 pattern. 



Palseontological evidence leads to the same conclusion. If the 

 known fossil American ancestors of the Hare be arranged in chrono- 

 logical order (Fig. 115), it is seen that as we go back in time the 

 molars are less hypselodont, the broad band of enamel which is con- 

 fined to the inner side of the teeth in Lepus spreads out over the 

 whole tooth, and in the earliest stage the vestiges of two roots are 



*Journ. Anat. a. Phyxiol., Vol. XXXVII., 1903, p. 144. 



-"Oil Fossil and Recent Lagomorpha," Trans. Linn. Soc. Loud. ('2), VII 1S99 

 pp. 433-520. 



