TRUE TEETH AND HORNY PLATES OP ORNITHORHYNCHUS. 31 
reached a very high degree of morphological and histological 
complexity, as I have shown in this paper and also in the pre- 
liminary account in the ‘ Proc. Roy. Soc.’ If, as Professor 
Seeley supposes, the horny plates are degenerate true teeth, 
every consideration points to the conclusion that the latter 
must be identified with the structures which I have described. 
Lastly, Professor Seeley considers that the anterior horny 
plates are still more degenerate and horny “ dental ridges,” 
which have become “ dental layers formed of vertical parallel 
plates of horn in which there is no division into separate 
teeth.” I shall presently show that the minute structure of 
the anterior plates cannot be described in these terms, and 
that they are neither more nor less horny than the posterior 
plates, but in every way identical with the latter in structure. 
Since Professor Seeley’s conclusions as to the structure and 
significance of the horny plates cannot be accepted, it is 
unnecessary to allude to any of the arguments founded upon 
such conclusions. 
Five years ago I prepared sections of the posterior horny 
plates, making use of some material kindly given me by Pro- 
fessor Moseley. On making cut and stained as well as ground- 
down sections, I saw at once that the structures were entirely 
epithelial, and that previous descriptions of the miuute struc- 
ture had been erroneous. I left the subject until last winter, 
when I again examined and figured some of the sections, 
intending to publish them with an account of the other 
epidermic structures of Ornithorhynchus. At Professor 
Lankester’s suggestion, however, I have added the description 
and figures of these structures to the present paper, because 
of Professor Seeley’s suggestions concerning them, and also 
because the posterior plates are evidently connected in some 
way with the fate of the true teeth, although structurally 
distinct from the latter. I have also prepared many more 
sections and have investigated the structure of the anterior 
plates. I am indebted to Dr. Hickson — Professor Moseley’s 
deputy, and to Mr. C. Robertson for kind assistance in the 
loan of specimens. The most valuable material, however, was 
