Mr. W. H. Flower on the Teeth in the Marsupialia. 131 



This change does not occur until the animal approaches the adult 

 age. 



5. DasyuridcB. — In a fcetal Thylacinus, in which no teeth had cut 

 the gum, the crowns of the permanent incisors, canines, premolars, 

 and anterior 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 evi- 

 dently its milk-predecessor. 



6. Phascolomijidce . — This family is placed last because the obser- 

 vations 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 toler- 

 able safety that the animals of the Order Marsupialia present a pe- 

 culiar condition of dental succession, uniform throughout the order, 

 and distinct from that of all other mammals. This peculiarity may 

 be thus briefly expressed. The 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 corresponding 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 INIammalia, in regard to the 

 mode of formation and succession of their teeth, into two groitps — the 

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

 Dijjhyodonts, or those that generate two sets of teeth ; but even in 

 the most typical diphyodonts the successional process does not ex- 

 tend 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 rudimentary diphyo- 

 dont 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 to the incisor teeth. It is questionable whether the first 

 premolar of those animals of this group which have four premolar 

 teeth, as the Hog, Dog (mandible), &c., ever has a deciduous pre- 

 decessor, at all events so f'ar advanced as to have reached the calci- 

 fied stage. But the closest analogy with the marsupial mode of 

 succession is found among the Rodents. Here the incisors appear to 

 have no deciduous predecessors ; and in the Beaver, Porcupine, and 

 others, which have but four teeth of the molar series, i. e. three true 



* The convenient distinction between false molars or premolars and true 

 molars, is always well marked in tlie form of the crown, especially in the upper 

 jaw, ill the Marsupials. 



9* 



