of the Rodentia to the Marsu2nalia. 291 



wliich fall out early in life, but the second pair is long- 

 retained. In nypsiprymnuH two incisors in the lower jaw 

 work against six in the upper, and of the latter the first pair 

 have grown considerably stronger, while the second and third 

 pairs are of inferior size. In Phalangista the six incisors 

 stand in an elegant horseshoe-like curve on the margin of the 

 broad premaxillse ; but in Hypsi'prymnus the snout has 

 become narrower, the premaxillfe being laterally compressed. 

 Then the four smaller incisors curve more towards the middle, 

 in order to function, as opposed to the upper teeth, in tearing 

 off ])lants. A large series of skulls of Hypsiprymnus shows 

 in what different ways this purpose can be attained. But the 

 four teeth are too weak to be retained with advantage in 

 adaptive groups ; hence they undergo the same fate as the 

 corresponding teeth in the lower jaw of Phalangista. In this 

 way it seems to me that the typical dentition of the Rodents 

 with its two pairs of incisors has been produced. The trans- 

 formation of the enamelled and root-bearing incisor into the 

 persistently growing gnawing- tooth furnished with an 

 enamel plate on one side only may also be easily traced in 

 the stem of the Marsupialia. In the group of the Lago- 

 morpha the dentition shows conditions which accord well with 

 my speculations. In the upper jaw, behind the gnawing- 

 teeth, the second pair of small incisors is quite pressed towards 

 the median plane ; they are also changed and have acquired 

 the power of persistent growth. The gnawing-teeth them- 

 selves, in both the upper and the lower jaw, also have very 

 short alveoli and a slight curvature. 



Notwithstanding the undoubtedly important part which 

 the dentition plays in rapid systematic diagnosis I do not 

 think that the notion of a direct blood-relationship can be 

 founded with sufficient certainty upon the similarity of the 

 dentition alone. Therefoie I will adduce further proofs. 



The horizontally inward projection of the angle in the 

 lower jaw of the Marsupials is well known as a very con- 

 venient and striking character. If the Rodents be pliylo- 

 genetically related to the Marsupials this structure must also 

 be still recognizable ; and in fact the comparison of many 

 skulls has shown me that the often-described bending of the 

 posterior angle of the inaudible in Rodents, which occurs in 

 variable degrees in different sections of the order, is derivable 

 in a direct series from what is found in the Marsupials. I 

 affirm most decidedly that Rodentia and Marsupialia manifest 

 their relationship by the homologous behaviour of the angle 

 of the mandible. In Muridaj, Hciuridas, and Myoxidas this 

 peculiarity is particularly clearly marked, although it has 



