638 
ME. W. H. FLO WEE ON THE DEVELOPMENT AND 
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*. 
Considerable differences occur in the various genera as to the relative period of the 
animal’s life at which the fall of the temporary molar and the evolution of its successor 
takes place. In some, as Hypsiprymnus, it is one of the latest, in others, as Thylacinus , 
and probably Pliascolomys , one of the earlier phenomena of dental development. 
Further observations on this point as opportunities occur will be interesting. 
As before stated, I regard the first tooth of the molar series of the young Kangaroo 
as one of the premolars of the permanent series, and not (like the tooth placed imme- 
diately behind it in the first stage of dentition) as a molar of the deciduous or milk- 
series. It is in fact the homologue of the penultimate premolar of Phalangista , 
Perameles , Pidelphys , &c. The circumstance of its being shed at a comparatively early 
period is in relation to the general conditions which, in this genus, cause the early loss 
of all the teeth between the incisor and true molar series, including the canine, and even 
the successional premolar. In Hypsiprymnus a still more potent reason prevails for its 
early removal, in the immense size of the successional premolar, which requires so much 
more space than is occupied by its diminutive actual predecessor. 
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 Monophyodonts , or those that gene- 
rate a single set of teeth, and the Diphyodonts, 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 poste- 
riorly in each series. 
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 each jaw. This position is, however, by no means without analogy among the 
mammals of the placental series. In the Dugong, and in the existing Elephants, the 
successional process is limited to the incisor teeth. It is questionable whether the first 
premolar of many of those animals which have four teeth of this group, as the Dog 
and Hog (mandible), ever has a deciduous predecessor, at all events so far advanced as 
to have reached the calcified stage. 
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 molars and one premolar, the latter is, exactly as in the Marsupials, the only tooth 
which succeeds a deciduous tooth. The analogy, however, no longer holds in those 
Rodents which have more than one premolar, as the Hare and Rabbit ; for in this case 
each of these teeth has its deciduous predecessor. 
* The convenient distinction between false molars or premolars and true molars, is well marked in the form 
of the crown, especially in the upper jaw, in all Marsupials. 
