1889.] 



On the Dentition of Oraithorhynchus. 



127 



1. Teetli are present, and they are typically mammalian in form 

 and structure. 



2. They are apparently functionless, being mere persistent rudi- 

 ments of teeth functional at an earlier stage of the animal's history, 

 and, judging from the material examined, are absorbed without ever 

 cutting the gum. 



3. The cornules are developed from the buccal epithelium covering 

 the teeth, and take their first shape from the dental cusps and ridges 

 beneath them. 



The grand fact of the presence of teeth in Monotremes, and their 

 mammalian nature, are discoveries on which Mr. Poulton may well be 

 congratulated, but, owing to the extreme youth of his specimens, the 

 inferences placed under the headings 2 and 3, well founded as they 

 then appeared to be, prove to require some modification. 



Thanks to the material before me, I am now enabled to carry- 

 forward our knowledge of the development of the teeth in their later 

 stages with somewhat greater certainty, but the fact that this know- 

 ledge has not been previously obtained affords a striking instance of 

 our ignorance of the most obvious and easily discovered points in 

 mammalian morphology. 



Re-stated according to the present observations, the paragraphs 

 above referred to should be — 



2. The teeth are functional for a considerable part of the animal's 

 life, cutting the gum as usual, and, after being worn down by friction 

 with food and sand, are shed from the mouth as are the milk teeth of 

 other mammals. 



3. The cornules are certainly developed from the buccal epithelium, 

 but from that under* and around, instead of over, the teeth, and the 

 hollows in the plates are the vestiges of the original alveoli of the 

 teeth, from out of which the latter have been shed. 



The material on which these conclusions are based is as follows : — f 

 a. The skull, extracted from the skin, of a young male Ornitho- 



* The idea of epithelium under a tooth seems at first sight an absurdity, since of 

 course the tooth is originally formed under the epithelium, and cuts through it on 

 being exserted. In this case, however, as a later stage, the epithelium appears to 

 push its way underneath the tooth, and, as the roots are absorbed, to obliterate the 

 holes in which they were situated, and, after the loss of the teeth, to form the even 

 surface of the hollows in the cornules. — May 17, 1889. 



f Since the above was written, Professor Stewart, of the College of Surgeons, has 

 discovered a set of teeth in a specimen in that museum. Of these teeth, ten 

 instead of eight in number, he is now preparing an account. Many details will be 

 found there as to the development and histology of the teeth and cornules that 

 could not be made out in the Museum specimens. I must at the same time express 

 my obligations both to Professor Stewart and Mr. Poulton himself for much advice 

 and assistance in making out the history of the teeth and cornules. — May 17, 

 1SS9. 



