PHYLETIC BRANCHES AMONG THE VERTEBRATES 173 



Plagiaulax that Lemoine has bestowed on it the 

 name of Neoplagiaulax, which implies a direct 

 ancestral relationship. The small Plagiaulacides of 

 Rheims only differs, in fact, from the Jurassic 

 genus by the reduction in number of the pre-molars, 

 which from four come down to one, this single pre- 

 molar becoming enormous while retaining the sharp 

 edge and the elegant oblique striae characteristic 

 of the branch. 



Finally, after a new disappearance for the whole of 

 Tertiary times, we have to seek the probable descend- 

 ants of the Plagiaulax in the Diprotodont Marsu- 

 pials (with two lower incisors only) of the Australian 

 Continent, such as the gigantic Thylacoleo of the 

 Quaternary of Queensland, or, perhaps, the small 

 kangaroo-rats or Hypsiprymnus, of which the long 

 conical incisor and the large pre-molar recall in a 

 striking manner the dentition of the Neoplagiaulax 

 of Rheims. The phyletic branch of the Plagiau- 

 lacidse thus presents all the features of slowness and 

 discontinuity of evolution which we have noted in 

 the most classic series of forms of the group of the 

 Invertebrates. 



We now arrive at the higher or Placental Mam- 

 mals, which have always been used as an argument, 

 seemingly decisive, in favour of a rapid evolution of 

 these different branches. It is certain that in the 

 present state of science leaving aside for the 

 moment the question, still vexed, of the Cretacean 

 Mammals of Patagonia the placental Mammals 

 appear, both in Europe and in the United States, 

 only in the very lowest strata of the Tertiary. The 

 faunas of Puerco and of Torrejon in America, and 



