296 PROCEEDINGS OF THE ACADEMY OF [1888. 



fiirtlier examination which developed the fact that all the specimens 

 of the oiiove list agree in the folloAving particulars: 1. In evidence 

 of the presence of three incisors, (No's 47744, 47739, 47743). 2, In 

 evidence, direct and indirect, of the presence of six j^i'^'^nolariform 

 teeth, (No's 47743, 47739, 47742). 3, In evidence of the pres- 

 ence of but three molariforni teeth. 4, In the fact that the mylo- 

 hyoid groove does not terminate at the dental canal but extends 

 back beneath the lower border of the pterygoid fossa, (No's 47751, 

 47754). All the specimens which do not directly bear upon these 

 four features of agreement support them indirectly, or at least pre- 

 sent no negative evidence. 



The formula, pm^, vi^, is very exceptional, and Mr. Oldfield 

 Thomas, who kindly examined these specimens and discussed the 

 dentition with the writer, suggested a different division of the series. 

 In No. 47739 the third and fourth premolars present lower crowns 

 than the succeeding tooth, fig. 1 b, but this is apparently because 

 the tips are not fully exposed. And as we have at present uo other 

 data than the mere form of the teeth, it seems that we are bound 

 to take the dentition as it stands, exceptional as it is, and divide 

 it provisionally as follows: 



The almost invariable presence of four premolars among the 

 Mesozoic and recent mammals is a very difficult fa-ct to explain. 

 This genus and apparently Ampliitlierium are among the few 

 exceptions. Why was the line drawn exactly to include five teeth 

 behind the incisor series, the first of these developing into a canine? 



In describing Peramus (op. cit., p. 2()3), the writer questioned the 

 reference of the anterior portion of the jaw, (No. 47743) to iton several 

 grounds, but now considers this less doubtful, as the single incisor 

 preserved is very similar to that in No. 47744., and both differ from 

 those of Stylodo)i, the only other type which this specimen resembles. 

 The last premolar has a heel very similar to that of the molars. The 

 molars, fig. 1 a, are very simihvr to those of some of the eocene Creo- 

 donta, presenting the primitive tuber cxdar- sectorial type. Among 

 the Jurassic INIammals, they apparently approach most closely the 

 molars of Amplutherlmn. 



AMPHITHERIUM. 

 A comparison of the three specimens belonging to this genus, two 

 in the Oxford collection, and the one previously studied in the 



