130 | Royal Society :— 
has been repeatedly subjected to examination, and described with 
exhaustive minuteness of detail, it is a singular cireumstance 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, although 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. Macropodide.—tThe dentition of the Kangaroo (genus Macro- 
pus), from the completely edentulous feetus 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 throughout 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 known, 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. Phalangistide.—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. Peramelide.—No very early stages of Perameles were examined ; 
but adolescent specimens of this genus and of Cheropus 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. Didelphide.—In the American genus Didelphys, the observa- 
tions are complete from the earliest stage, and show that, as in the 
Australian Macropodide and Phalangistide, 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. 
