TEUE TEETH AND HOENY PLATES OF OENITHOEHYNOHUS. 9 
The True Teeth and the Horny Plates of 
Ornithorhynchus. 
By 
Edward U. Poulton, M.A., F.L.S., 
Of Jesus aud Keble Colleges, Oxford. 
With Plates II, III and IV. 
Part I. — The True Teeth. 
Historical — Number of the Teeth — Position of the Teeth — Form of the Teeth 
— Structure of the Teeth : 1 . Tooth-papilla or Dentine Germ ; 2. Dentine ; 
3. Euamel ; 4. Inner Epithelium of the Enamel Organ ; 5. The Stratum 
Intermedium of Hannover; G. The Middle Membrane of the Enamel 
Organ ; 7. The Outer Membrane of the Enamel Organ — The Less De- 
veloped Fourth Tooth — Conclusions — Future Investigations — Conclusions 
of Other Writers. 
Historical. — The mature Ornithorhynchus has always been 
described as without true teeth. It is well known to possess eight 
horny plates, two upon each side of each jaw. The true teeth 
are developed at an early stage beneath the posterior horny 
plates, so that some connection between the fate of these latter 
and that of the true teeth will very probably be found to exist, 
when material can be obtained. 
Although no direct observations have been hitherto made 
upon the subject of this part of the present paper, several writers 
have argued that the ancestors of the MonoHemes must at one 
time have possessed teeth. All who follow Hertwig’s identifi- 
cation of the mammalian tooth with the placoid scale must 
believe by implication that this was the case, for the Mam- 
malia must have received their true teeth through those an- 
