20 EVOLUTION OF MAMMALIAN MOLAR TEETH 



Dromathcrium, which is very stout and convex with a thick lower border, 

 projecting widely from the matrix, an elevated coronoid process, while the 

 curvature of the lower border is unbroken by any downward projection. 

 If these differences may be given merely a specific value, and attributed 

 in part to the fact that the Microconndmi jaw is seen upon the outer 

 surface, and that of Dromathcrium upon the inner surface, let us compare 

 closely the teeth in the two genera. Unfortunately the canine and 

 incisors of the Microconodon ranius are wanting. We first observe that 

 the premolars of Dromathcrium are styloid and procumbent ; if erect 

 they would rise above the level of the molars ; they have no trace of a 

 cingulum. In the other genus the premolars are subconical, and, although 

 erect, they do not reach the level of the molar tips ; they show a faint 

 posterior cingulum, and the third premolar has the same evidence of a 

 division of the fang which is seen in the molars, while in Dromatlurium 

 there is no trace of such a depression, but a distinct groove on the 

 postero-internal face of the tooth reaching nearly to the summit. The 

 molars of Dromathcrium are narrow and loft}-; the general pattern of the 

 crown consists of a single main cone with a high anterior and lower 

 posterior accessory cusp upon its slopes; but these cusps are very irregular 

 in disposition. For example, in the second molar there are two anterior 

 cusps ; in the third molar the posterior cusp is nearly as large as the 

 main cusp : in the fifth molar there is a trace of a postero-external cusp ; 

 in the last molar both the anterior and posterior cusps are distinctly bifid 

 at the tip. In Microconodon, on the other hand, the molars are com- 

 paratively low and broad, with a low anterior and higher posterior 

 accessory cusp ; these cusps are regular and very prominent ; there is also 

 a well-marked posterior cingulum, which cannot be distinguished in the 

 corresponding molars of the other genus. 



Although the two posterior molars are wanting in Microconodon, the 

 rise of the coronoid probably marks the position of the last molar ; taking 

 this estimate of the posterior point of the molar-premolar series and 

 comparing it with the length of the series in Dromathcrium, we find that 

 while the ramus of one genus is only two-thirds the length of the other, 

 the total space occupied by the molar-premolar series is very nearly the 

 same. Estimated in another way, the molar-premolar series of Micro- 

 conodon is a little less than one-half the entire length of the jaw (y 6 ^), 

 while that of the other genus is exactly one-third the length of the jaw. 

 This discrepancy is due to the difference in the proportions of the molars; 

 in one genus they are low and broad at the base, in the other they are 

 unusually high and compressed. 



It is difficult at present to assign any systematic position to either of 

 these genera. Dromatherium is entirely unlike any known mammal, 

 fossil or recent. The form of the molars is extremely primitive both in 



