TRUE TEETH AND HORNY PLATES OF ORNITHORHYNCHTJS. 23 
attained by this layer in the enamel organs of the simple and 
complex teeth of the same species of animal. At the same 
time there is as yet no proof that the simple teeth of Mammalia 
have not possessed a more complex form at some stage in 
their developmental history. 
In Ornithorhynchus the layer is much less developed in the 
simpler first teeth than in the more complex posterior ones 
(compare a, figs. 1 — 3 with n and c, PI. II). On the 
other hand, the former are more developed and the layer 
may have been somewhat reduced in size. It nevertheless 
contains abundant blood-vessels. 
Finally, the existence of such highly developed teeth in the 
posterior part of the jaws, and the absence of any traces 
anteriorly, at any rate in specimens of the age examined, 
seem to clearly indicate that the bill of Ornithorhynchus is a 
very ancient structure, if not in its present form, at least as 
some kind of horny beak which could take the place of anterior 
teeth. I think, however, that it is very probable that the 
rudiments of teeth may be found anteriorly at a much earlier 
stage, when the bill is less developed than was the case in Dr. 
Parker’s specimens. 
Future Investigations. — In addition to the last-men- 
tioned point, other questions which require investigation are 
the epithelial nodule in the stellate reticulum, the extent of 
the capillary network in the latter, the possibility of any 
further development of the teeth in later stages and their 
relation to the horny plates, and the presence of an anterior 
tooth in the lower maxilla. I am now working upon the 
young stages of Echidna and of the toothless Edentata, and 
hope to shortly publish an account of the results. 
Conclusions or Other Writers. 
Since the appearance of the preliminary note in the ‘ Proc. 
Roy. Soc.’ three writers have published their opinions as to 
the conclusions which may be drawn from the presence of true 
teeth in Ornithorhynchus. 
