ORDINAL TYPES OF Mnl.AKS: IIODKXTIA 



145 



these animals are not Rodents lut Lemurs related to the aberrant 

 order represented by the living Cheirom//* or Aye-Aye. The evidence 

 for either of these antagonistic hypotheses is by no means final. It' 

 these animals are truly primitive Rodents or I'roglires, they settle the 

 tritubercular question so far as Rodents are concerned, bemuse the 

 teeth are typically tritubercular above and tuberculo-sectorial below 

 (Fig. 104). Pending the positive discovery of the remote ancestor- 

 of the Rodents, it may be said that the most primitive existing and 

 fossil forms of brachyodont rodents, as the Eocene and Oligocene Ischy- 

 romyidse, Sciuridse, exhibit apparent traces of the tritubercular pattern 

 in the molar teeth. 



FIG. 104. Tritubercular molars in the " Proglires," possibly related to the Rodents. Upper 

 figure, an upper molar of Olbodotfg copei from the Torrejon Formation, Stage II. Basal Eocene, 

 showing a primitive tritubercular crown, with a hypocone growing up from the basal congulum. 

 Lower figure, lower jaw and teeth of Mi.codectus pungcns also from the Torrejon Formation, 

 showing enlarged incisor and tuberculo-sectorial molars. See note 3, page 144. x ?. 



SlMPLICIDEXTATA. 



The animals included within the sub-order Simplicidentata (i.e. 

 with a single pair of incisors, as contrasted with the Duplicidentata, 

 or Rabbits and Hares) are traced back by Tullberg in his great 

 monograph, 1 to an ancestral type in which the molars exhibit four 

 cusps. Schlosser 2 also has figured a morphological series of upper and 

 lower molars, showing the probable stages of evolution from the bunodont 

 tritubercular (?) molars of Ardomys to the hypsodont complex molars of 

 Hystrix. Forsyth Major, on the contrary, who has made an exhaustive 

 study of the teeth of Rodents, regards the teeth of primitive squirrels, 

 which are apparently tritubercular, as secondarily derived from a 

 polybunous form by the loss of certain cusps. It is more consistent 



1 Ueler das System der ^"ayethiere. Upsala, 1S99. 



2 "Die Differenzierung des Saugetiergebisses," Biol. Ctntrti/1,/., Bd. X., Nr. 8, 1890, 

 S. 251. 



K 



