1888.1 of Reptilian Character in Mammalian Teeth. 133 



cause. With simplicity of function there is simplicity of detail in the 

 crown of the tooth. Some of the simplest teeth are found among the 

 Edentata, where the tooth is often sab-cylindrical, but as the crown 

 is worn down, its original form is not seen. Tatusia, however, is an 

 Edentate with successional teeth, and while the crown is still within 

 the jaw it has a form which is as reptilian in aspect as the molar tooth 

 of a Teius. The crown of the tooth of a Cachalot is a short curved 

 cone. Hence it is manifest that the molar teeth of mammals are not 

 necessarily cuspidate, and that in simplicity of crown there may be 

 no character to distinguish a mammal from a reptile. From which it 

 is probable that some primitive fossil mammals may also have a rep- 

 tilian type of dentition. The recent discovery of a set of teeth in the 

 jaws of Omithorhynchus , hitherto unknown, raises the question whether 

 those teeth are mammalian. Mr. Poulton has only contributed a 

 vertical transverse section of one of these teeth,* which shows elevated 

 external and internal cusps. I have no other knowledge of those teeth, 

 but the condition figured is suggestively similar to a corresponding 

 section of a molar tooth of the lizard genus Teius.f Professor Mivart 

 quotes^ from Mr. Poulton a passage, which I do not find in that 

 gentleman's paper, describing the tooth, and from that description it 

 would appear to correspond generally with the tooth of the adult 

 Omithorhynchus. I have already considered some characters of those 

 teeth, and allowing for their degeneration, they seem to me to 

 approach as close perhaps to the form of crown in lizards like Teius 

 as to any of the larger bats. 



Fifthly, mammalian teeth are often remarkable for the wear of the 

 crown. This attrition appears to depend upon the form of the crown, 

 the apposition of crowns, the development of enamel, and the nature of 

 food. It is exceptionally well seen among Elephants, Ungulates, and 

 Edentates ; but almost all mammalian teeth show some change of 

 aspect with wear. This condition is much less general among reptiles ; 

 but in the extinct Ornithischia the serrated crowns of the teeth are 

 as well worn as in any mammal. The long teeth of Hyperodapedon§ 

 appear to be well worn down to the palate. Exceptionally teeth of 

 Ichthyosaurus and Polyptychodon show both vertical wear and lateral 

 wear, and there are specimens in the Woodwardian Museum from the 

 Cambridge Greensand in which teeth of these genera have the crown 

 worn away transversely almost down to the root ; so that neither 

 wear nor its absence has any importance as a class character, 



* 'Roy. Soc. Proc.,' vol. 43, p. 355. 



f Sir It. Owen compares the teeth of Omithorhynchus to those of the reptilian 

 fossil Placodw (' Geol. Soc. Quart. Journ.,' vol. 3(5, p. 423), but the details of 

 structure of the crown are not the same. 



X ' Roy. Soc. Proc.,' vol. 43, p. 373. 



§ Lydekker, ' India Geol. Surr. Mem.,' ser. iv, vol. 1, part 5, pi. 2. 



