1889.] 



On the Dentition of Ormthorhynchus. 



131 



ship nearer to each other than to any other known groups of 

 mammals. 



In any case, since the form and structure of the teeth are of neces- 

 sity the chief means of determining the evolutionary history of the 

 Mammalia, the discovery now made, in giving us gennine modern 

 Monotrematous teeth to work from, provides one of the most im- 

 portant aids to the elucidation of the systematic position of these 

 anomalous mammals that has yet been obtained. 



Finally, it may be noted that the absolute continuity of the epithe- 

 lium with the developing cornule, combined with the presence of 

 such well-developed calcined teeth, proves again, if after Mr. Poulton's 

 paper further proof is needed, that the view* as to the cornules being 

 degenerated true teeth is wholly untenable. 



EXPLANATION OF PLATE 2. 



Fig. 1. Left upper teetli of OrnitliorJiynelius ( x 5). Drawn from specimen a (see 

 supra) ; the rim of indurated epithelium still present. 



2. Left lower teeth ( x 5) . 



3. Left side of palate of specimen b, showing (a) the empty anterior alveolar 

 hollow, (b) the worn-down posterior tooth not yet shed, and (c) the elevated 

 rims of epithelium that would later have formed the walls of the ccrnules. 



4. The same in the lower jaw, but here all the teeth have been shed : c as in 3. 



5. Molar tooth of Microlestes, much magnified. 



6. Series of diagrammatic sections showing development of tooth and cornule. t 



a. Tooth (t) still in capsule below gum; e, epithelium. This sketch is 



taken from one of Mr. Poulton's figures of the early stages of the teeth. 



b. Tooth just before eruption. 



c. Eruption of tooth and consequent cutting of the epithelium, which com- 



mences to thicken at e for the formation of the cornule. 



d. Creeping of epithelium underneath tooth, until it presses against and 



gradually causes absorption of their roots. 



e. Tooth just before it is shed. Its roots have been absorbed, and the 



epithelium has passed right beneath it. The cornule is now definitely 

 separated (at x) from the ordinary epithelium, and its edges are so 

 developed as to overtop the cusps of the tooth. 



f. Fully developed cornule. 



* Seeley, ' Roy. Soc. Proc.,' vol. 44, 1888, p. 129. 



f These diagrams were drawn up in consultation with Mr. Poulton, so that he is 

 equally responsible for them with myself. 



