1892.] 



FROM THE OLIGOCENE OF ITALY. 



79 



external wall, the anterior root of the zygomatic arch, a portion of 

 the floor of tlie orbit, and the aperture of the canal for the fifth 

 nerve. At the hinder extremity of the palatal surface are preserved 

 two molariform teeth, which are almost or quite untouched by wear, 

 and are, fortunately, uninjured. By filing away the alveolar parapet 

 in front of the anterior one of these teeth, there have been revealed 

 the two roots of a third tooth. Behind the last remaining tooth 

 there is a portion of the alveolus of a fourth touth which had 

 evidently not been protruded from the gum. 



Now the unworn condition of the two remaining teeth and their 



Figs. 1, 1 a, 1 b. — The third and fourth left upper milk-molars of a young 

 individual of Prorastonm mrojienae, viewed from the oral and outer 

 aspects ; 1 fl' is |, the others y. pr., protocone ; j3«., paracone ; 

 me., metacone ; hij., hj'pocoue. The letters a, 6, c, tZ, indicate the 

 parts of the hinder tooth which correspond with those similarly 

 lettered in fig. 2. 



Fig. 2. — The third left ujjper molar of Merycojjotamus dissimilis. Letters 

 as in preceding figure. \. 



position with regard to the anterior root of the zygomatic arch, 

 coupled with the alveolus of the unprotruded fourth tooth, clearly 

 indicate that the jaw belonged to an exceedingly young animal. 

 And, apart from their structure, we should further he justified in 

 regarding the two entire teeth as belonging to the deciduons series if 

 their owner were a diphyodont mammal, or to the premolar series if 

 the animal were monophyodont. There are, however, considerations 

 which enable us to determine the serial homology of the teeth more 

 definitely. 



Each of the two teeth carries on its crown well-marked but dis- 

 continuous transverse ridges, separated by deep valleys. In the 

 hinder tooth the crown is oblong and nearly syraraetricalj and carries 



