TRUE TEETH AND HORNY PLATES OF ORNITHORHYNOHUS. 37 
concavity of the upper plate may be similarly formed over the 
small anterior tooth, or it may represent the epithelial ridge in 
front of the true teeth. But it must be remembered that the 
latter is external in position while the concavity is internal. 
Furthermore, the concavity has its special alveolar compart- 
ment, which seems to indicate the former existence of a true 
tooth. On the other hand, the posterior small concavity of 
the lower plate is a difficulty on this latter interpretation, for 
it possesses its compartment, and yet it arises in a position 
where no small true tooth developes ; for it is unlikely that 
the very rudimentary tooth-germ attains any degree of spe- 
cialisation ; furthermore, there is a corresponding germ in the 
upper jaw, and yet no concavity. It seems on the whole pro- 
bable that the alveolar compartments of the small concavities 
of both upper and lower plates are simply parts of the alveoli 
for the second and third true teeth respectively, and that the 
simple anterior tooth does not impress itself on the plate, or, if 
so, does not produce any effect which can be distinguished from 
that of the large tooth behind it. Again, the small concavities 
may be due to subsequent differentiation of the plates. It is 
quite clear that we cannot be sure as to the correct interpretation 
of details, although they will be settled with certainty when 
more material is obtainable. Thus it is certainly possible that 
the epithelial ridge shown in PI. IV, fig. 13 may have nothing to 
do with the plates, and that the latter originally arose over the 
true teeth only, in the manner described above. But, under 
any circumstances, the subsequent history appears to be toler- 
ably clear. The true teeth, after ceasing to come to the sur- 
face, would be absorbed at successively earlier stages, thus 
permitting the horny plates to gradually intrude into their 
alveoli, so that in the adult animal the bone and the under 
surface of the epithelium are everywhere in close proximity. 
Many sections in various directions through both upper and 
lower plates in their sockets failed to reveal any traces of the 
true teeth, so that absorption is probably complete. The 
contour of the surface of the plates, originally determined by 
the underlying teeth, would still be maintained as far as 
