84 Mammalia. Digestive System. 



arity may be thus briefly expressed. The teeth of Marsupialia do 

 not vertically displace and succeed other teeth, ivith the exceptionjof 

 a single tooth on each side of eachjaiv. 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 pre- 

 ceded by a tooth having the characters more or less strongly ex- 

 pressed of a true Molar. If you divide the Mammalia in regard to 

 the succession of teeth into Monophyodonts and Diphyodonts, the 

 Marsupials occupy an intermediate position, presenting as it were a 

 rudimentary Diphyodont condition, the successional process being 

 confined to a single tooth on each side of the jaw; to which, how- 

 ever, analogous examples may be met with in the Placental series 

 in the Dugong and Elephant, in which the successional process is 

 limited to the Incisor teeth ; and in those members of the Eodentia 

 which have but four teeth in the Molar series, i.e., three true Molars 

 and one premolar {e.g. Beaver, Porcupine), in which the latter is, 

 exactly as in the Marsupialia, the only tooth which succeeds a 

 deciduous tooth*. Number, Incisors are present in all the species, 

 but are variable in number ; in some genera {e.g. Peramelidse and 

 Didelphidae) of the flesh-eaters, exceeding that of the Mammalian 

 type in the upper jaw (e 5 . 5). Canines are present in all the 

 flesh-eaters, but are not constant in the plant-eating genera, being 

 absent in the Phascolomydae and Macropidae (except in upper jaw 

 of Hypsiprimnus, and the foetal Macropus ; in the latter they are 

 never functional), and but feebly represented in the Phalangistidse 

 (absent in lower jaw of Phascolarctos). The typical number of teeth 

 in the Molar series is seven on each side of both jaws. Those pos- 

 terior teeth of either side of each jaw which have no deciduous pre- 

 decessors are as a general rule, four in number, instead of three, as 

 in most Placental Mammalia ; but in Myrmecobiidae the number of 



true (m ^-~^) and false Molars is eighteen in each jaw, exceeding 



that of any other known existing Marsupial {cf. supra ; Insectivora ; 

 Chrysoclore) ; and Petaurus pymaeus is said to have only three true 

 Molars on either side of each jaw. The dental formula of the Phasco- 

 lomydae is the same both in number and kind to that of Eodentia. 

 The dental system of the Dasyuridae corresponds generally with that 

 of the Sectorialia ; it diff'ers in having the Incisors in greater num- 



Flower. Phi^, Trans. 1867. 



