130 Royal Society : — 



has been repeatedly subjected to examination, and described with 

 exhaustive minuteness of detail, it is a singular circumstance that 

 most of those peculiarities in the succession of their teeth which 

 distinguish them from other mammals appear hitherto to have escaped 

 observation. To supply this blank is the object of the present com- 

 munication. Fortunately the materials at my disposal, althovigh not 

 quite so complete as might be desired, are yet amply sufficient to 

 illustrate the main aspects of the question, and to supply a result 

 as interesting as it was unexpected. 



Descriptions are given in the paper, accompanied by drawings, of 

 several stages of the dentition of members of each of the six natural 

 families into which the order is divided. 



1 . Macropodidce. — The dentition of the Kangaroo (genus Macro- 

 pus), from the completely edentulous foetus to adult age, is described 

 in detail. Contrary to what has been specially stated with regard 

 to this genus, there are no deciduous or milk-incisors, the teeth of 

 this group which are first formed and calcified in both jaws being 

 those which are retained througbout the life of the animal. The 

 rudimentary canine and first premolar have also no deciduous pre- 

 decessors. The second tooth of the molar series (a true molar in form) 

 is vertically displaced by a premolar. The four true molars have, as 

 has long been knovin, no deciduous predecessors. There is thus but 

 one tooth on each side of each jaw in which the phenomenon of di- 

 phyodont succession occurs. The period at which this takes place 

 varies in different species of the family. In some forms of Hypsi- 

 prymnus the successional premolar is not cut until after the last true 

 molar is in place and use, — this probably having relation to the ex- 

 traordinary size of the tooth, and the time consequently required for 

 its development. A special characteristic of this family is the ten- 

 dency to lose the canine and one or both premolars at a compara- 

 tively early period of life, 



2. FhalanyistidcB. — Several early stages of the dentition of Pha- 

 langista vulpina are described and figured. In a young specimen in 

 which no teeth had cut the gum, the crowns of the permanent in- 

 cisors, canine, and first two molars were found to be calcified, and 

 the germ of the permanent premolar was already formed beneath the 

 milk- or deciduous molar, which, as in Macropus, is the only tooth 

 which is shed and replaced by a successor. The change takes place 

 at an earlier period than in the last family, 



3. Peramelidce. — No very early stages oi Perameles were examined; 

 but adolescent specimens of this genus and of Choeropus show that 

 a very minute, compressed, molariform tooth is' replaced by the tri- 

 angular, pointed, third or posterior premolar. No other signs of 

 vertical displacement and succession were observed, 



4. BidelphidcB. — In the American genus Bidelphys, the observa- 

 tions are complete from the earliest stage, and show that, as in the 

 Australian Macropodidce and Phalanyistidce, none of the teeth of the 

 permanent series have predecessors except the compressed pointed 

 last premolar, which replaces a tooth having the broad multicus- 

 pidate crown of a true molar. 



