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that this is a secondary condition derived by reduction from a diphy- 
odont stage, still more recent research proves that we must regard all 
Mammals as potentially polyphyodont and possessed of traces of 4 or 
5 dentitions. 
Placing on one side Röse’s (1) placoid dentition, concerning the 
presence of which in the mammalian phylum we may feel a little 
doubtful, we have still 4 dentitions left, which may be thus roughly 
described : 
1st or premilk dentition, 
2nd or milk dentition, 
3rd or replacing (permanent) dentition, 
Ath or postpermanent often described as the ärd dentition. 
The first of these was originally described by LEcHe (2, 3, 4, 5) 
from the discovery of certain growths of the dental lamina situated 
labial to the true milk teeth in Erinaceus. Somewhat similar 
structures had been described as long ago as 1866 by HERTZ (6) in 
the Ox and Röse (7) had also found them in man, but although these 
structures are now regarded by the latter as homologous with LECHE’s 
premilk dentition, yet neither of these two authors originally inter- 
preted them as such. 
LEcHE was able to confirm his original suggestion by the discovery 
in Myrmecobius (8 and 4) of several minute calcified teeth situated 
labial to and preceeding the functional teeth of that animal in order 
of development, and as this latter set had, by his own researches and 
those of KUKENTHAL (8) and Röse (9), been shown to correspond with 
the milk set of the Placentalia, he concluded that these calcified struc- 
tures represented a premilk dentition. LEcHE also describes (4) what 
he believes to be a premilk vestige in Didelphys in connection with 
i1. To this same order Röse (11) following LEcHE (5) would now 
refer the minute calcified teeth which he described in the Wombat (10), 
and I also am inclined to beleive that the three pairs of rudimentary 
upper incisors which I have described (12) as present in the young 
of several genera of the Macropodidae are correctly referred as LECHE (4) 
has done to the same premilk dentition. 
The most recent addition to our knowledge of the premilk den- 
tition is to be found in a paper by Röse (13) on the dentition of the 
Ox where he describes several labial outgrowths from the dental la- 
mina which he regards as premilk vestigies. 
For some years past I have been working at the polyprotodont 
Marsupials and have found thus minute set of teeth present in several 
genera and aS so much interest is attached to the presence of this set 
