II MONOPHYODONT DENTITION 5 
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which arise on the outer side of the fold of epithelium and some 
way above its lower termination. These ultimately acquire a 
bell-lke form, and are as it were moulded on to a thickened con- 
centration of the dermis beneath; they then become separate 
from the downgrowth of the epithelium whence they have arisen. 
Finally, each of the eight germs becomes one of the milk teeth of 
the animal. The lower end of the sheet of invaginated epi- 
thelium, the common enamel germ, is the seat of the formation 
of the second set of teeth, of which, however, in the animal under 
consideration, there are only two in each jaw. But corresponding 
to each of the enamel germs of the milk dentition, with the 
exception of the first two molars, there is a sheht thickening of 
the end of the common enamel germ, which at a certain stage is 
indistinguishable from the thickening which will become one of 
the permanent teeth. We have thus the diphyodont arrange- 
ment. But this does not exhaust the series of rudimentary teeth, 
though no more come to maturity than those whose development 
has already been touched upon. In the upper jaw a small out- 
growth of the common enamel germ arises above and to the 
outer side of the enamel germ of the third milk incisor; this does 
not develop any further, but its resemblance to the commencing 
germ of a tooth seems to indicate that it is the remnant of a 
tooth series antecedent to the milk series. Furthermore, there 
are indications in the fourth premolar of a fourth series of teeth 
posterior in appearance to the permanent dentition. We arrive 
therefore at the important conclusion that although here as 
elsewhere there are only two sets of calcified teeth ever developed, 
there are feeble though unmistakable remains of two other series, 
one antecedent to and the other posterior to the diphyodont 
dentition. The gap therefore which separates the mammalian 
dentition from that of reptiles is less than has hitherto appeared. 
Dr. Leche also carefully studied the tooth development of Jguana ; 
he found that in this lizard there are four series of teeth which 
come to maturity, and a rudimentary series antecedent to these 
which never produces fully formed teeth. 
In a few mammals there is a kind of dentition known as the 
monophyodont, in which only one series of teeth reaches maturity ; 
where in fact there is no replacement of a milk series by a per- 
manent dentition. Of the monophyodont dentition Whales form 
an example. The Marsupials are very nearly an instance of the 
