1867.] and Succession of the Teeth in the Marsupialia. 465 



adolescent specimens of this genus and of Chceropus show that a very 

 minute, compressed, molariform tooth is replaced by the triangular, pointed, 

 third or posterior premolar. No other signs of vertical displacement and 

 succession were observed. 



4. Didelphidce. — In the American genus Bidelphys^ih^ observations are 

 complete from the earliest stage, and show that, as in the Australian Ma- 

 cropodidce and Phalangistidee, none of the teeth of the permanent series 

 have predecessors except the compressed pointed last premolar, which 

 replaces a tooth having the broad multicuspidate crown of a true molar. 



This change does not occur until the animal approaches the adult age. 



5. BasyuridcB. — In a foetal Thylacitius^ in which no teeth had cut the 

 gum, the crowns of the permanent incisors, canines, premolars, and ante- 

 rior true molars were partially calcified, and necessarily much crowded 

 together in the jaw. A very minute rudimentary molar was situated just 

 beneath the alveolar mucous membrane, superficially to the apex of the 

 hindermost premolar, and was evidently its milk-predecessor. 



6. Fhas6olomyid<sB. — This family is placed last because the observations 

 regarding it are less complete than in the case of any of the others. The 

 youngest Wombat available presented no evidence of succession of any of 

 the teeth ; but it is probable that the single premolar is preceded by a 

 milk-molar, at a still earlier period than any examined. 



From the foregoing observations it may be concluded with tolerable 

 safety that the animals of the Order Marsupialia present a peculiar condi- 

 tion of dental succession, uniform throughout the order, and distinct from 

 that of all other mammals. This peculiarity may be thus briefly ex- 

 pressed. Th€ teeth of Marsupials do not vertically displace and succeed 

 other teeth, with the exception of a single tooth on each side of each jaw. 

 The tooth in which a vertical succession takes place is always the corre- 

 sponding or homologous tooth, being the hindermost of the premolar series*, 

 which is preceded by a tooth having the characters, more or less strongly 

 expressed, of a true molar. 



It has been usual to divide the class Mammalia, in regard to the mode of 

 formation and succession of their teeth, into two groups — the MonophyodontSf 

 or those that generate a single set of teeth, and the JDiphyodonts, or those 

 that generate two sets of teeth ; but even in the most typical diphyodonts 

 the successional process does not extend to the whole of the teeth, always 

 stopping short of those situated most posteriorly in each series. The 

 Marsupials occupy an intermediate position, presenting as it were a rudi- 

 mentary diphyodont condition, the successional process being confined to a 

 single tooth on each side of each jaw. This position, however, is by no 

 means without analogy among the mammals of the placental series. In 

 the Dugong and the existing Elephants the successional process is limited 



* The convenient distinction between false molars or premolars and true molars, is 

 always well marked in the form of the crown, especially in the vipper jaw, in the 

 Marsupials, 



