Osbom.] J- J- 'J [April 15, 



Microconodon ramus are wanting. We first observe that the premolars 

 of Dromatherium 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 Dromatherium 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 Dromatherium are 

 narrow and lofty ; 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 exam- 

 ple, 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 Microcono- 

 don, on the other hand, the molars are comparatively low and broad, with 

 a low anterior and higher posterior accessory cusp ; these cusps are regu- 

 lar 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 corouoid probably marks the position of the last molar ; taking 

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

 paring it with the length of the series in Dromatherium, we find that while 

 the ramus of one genus is only two-thirds the length of the ether, the total 

 space occupied by the molar-premolar series is very nearly the same. Esti- 

 mated in another way, the molar-premolar series of Microconodon is a little 

 less than one-half the entire length of the jaw {f^), 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 com- 

 pressed. 



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

 these genera. Dromatherium is entirely unlike any known mammal, fos- 

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

 respect to the incomplete separation of the fangs and the remarkable varia- 

 tions in the number and size of the accessory molar cusps. In fact the 

 molars appear to be in what may be called an experimental stage of struc- 

 ture. The accessory cusps are sometimes large and distinct, as in the third 

 true molar ; sometimes minute like needle points, as in the second molar. 

 The incomplete separation of the fangs is a reptilian character, which 

 correlated with the styloid premolars and recurved canine-incisor series, 

 place Dromatherium very remote from any of the known Mesozoic mam 

 mals. Microconodon, on the other hand, is a somewhat more "recent" 

 type, the premolars have the trace of a low posterior heel, and the molars 



