See oe 
582 MR. M. F. WOODWARD ON [May 5, 
more constant in connection with the lst molars, yet were also 
found in the region of the 2nd molars in several genera. 
The lingual continuation of the dental lamina was found in all 
six genera examined, whereas the labial growth was more irregular 
and only observed in three forms; this latter growth was most 
conspicuous in Hrinaceus, where it was constant from the beginning 
of m. 1 to the end of m. 2, 
A great deal of stress has been laid upon the presence of these 
structures, especially that of the lingual one, its presence having 
been said to prove that the molar teeth belonged to the milk- 
dentition. There is no doubt that if we simply compare such a section 
as fig. 26 (Plate XXVI.), representing the molar tooth of Centetes, 
with a developing milk-tooth which is known to have a successor, 
we should certainly conclude that the lingual growth of the dental 
lamina in the two cases was the same structure; and as it can in 
one case be shown to give rise to the enamel-organ of a replacing 
tooth, we might apparently be justified in concluding that in the 
case of the molar it represented a reduced enamel-germ of a 
permanent tooth, and that the molar tooth belonged in consequence 
to the milk-dentition. But it is now well known that we have 
in the Mammalia traces of three or four sets of teeth; and as it is 
highly probable that the Mammalia are derived from polyphyodont 
ancestors, it is possible that there might at any time appear traces 
of a polyphyodont dentition. It appears, then, to me that presence 
of a lingual] continuation of the dental lamina does not necessarily 
imply that the labial tooth belongs to the milk-series; it might 
equally well belong to the permanent orto the post-permanent series, 
all traces of the earlier lubial sets being lost, the lingual growth 
being not merely the enamel-germ of a successor, but the free end 
of the undifferentiated dental lamina, which may go on growing 
and producing fresh sets of teeth, as it does in the polyphyodont 
reptile, where it is the “ anlage” of numerous enamel-organs. 
Compare for a moment these two diagrams (p. 583): fig. 1 repre- 
senting a section of the dental lamina of a reptile with a practically 
unlimited succession, while fig. 2 represents the milk-tooth of a 
mammal with a Jingual development of the dental lamina, which is 
here known to give rise to a permanent tooth; we should not in 
this case be justified in concluding that ‘“‘m” in fig. 2 was the 
homologue of 3 in fig. 1, merely because of the presence of this 
similar development of the dental lamina on its lingual side; we 
must either conclude that m is the homologue of 1 and the 
permanent tooth of 2, or perhaps 1 or both 1 and 2 have been 
completely suppressed, and therefore m is the homologue of 2 or 3 
as the case may be. In fact we must start with the dental lamina 
from the gum, looking most carefully for labial rudiments, so as to 
be perfectly sure that none of the earlier sets of teeth have 
disappeared, before we can homologize the functional teeth, and we 
must naturally expect to find a lingual growth of the dental lamina 
constantly present, whether we are dealing with the Ist, 2nd, or 
3rd sets, there being no reason to believe that there is an ultimate 
