374 



Prof. St. G. Mivart, On the 



[Feb. 16, 



(whicli appears to become mack worn down by use) with a small 

 accessory prominence both, on the inner and the outer side of the 

 central one. As every one knows, reptilian teeth, may become 

 obtuse rounded structures as in Cyclodus and Ada, or almost quite 

 flattened as in the curious extinct reptiles Lepidotus and Placodus. 

 The Theriodontia* offer examples of teeth more or less like the 

 incisors and canines of mammals, but exhibit no grinding molar, the 

 subdivisions of the summits of their molar teeth sometimes, however, 

 reminding us of the tricuspid molars so common in existing 

 Lacertilia.ns. 



Such being the negative evidence with respect to the molar teeth of 

 the Sauropsida, I availed myself of the kind assistance of Mr. Oldfield 

 Thomas, E.Z.S., in an endeavour to find amongst mammals teeth 

 like those described as existing in the Ornithorhynchus. Although 

 various forms were seen to present slight resemblances, we failed to 

 obtain any which could be said to bear an unquestionable likeness to 

 them. 



The ancestors of the Ornithorhynchus which had functional teeth, 

 must, according to the ordinarily received doctrine of evolution, have 

 had a general bodily organisation at least as Sauropsidan as that of 

 the existing Ornithodelphia. How far back in geological time that 

 tooth structure existed, we have as yet no evidence ; but we have 

 abundant evidence that a dentition much like that of some existing 

 Marsupials already existed during the deposition of the Oolite 

 strata. Professor Huxley has expressedf his expectation that, 

 generalised ancestors of the Monotremes may be found amongst the 

 remains "of the terrestrial Vertebrates of the later Palaeozoic 

 epochs." 



The toothed ancestor of the Ornithorhynchus, however, could I 

 think hardly have been extant at so extremely distant an epoch ; for 

 then its resemblance in other respects to the Lacertilia would make it 

 probable that it had a pretty close connexion with the stem of the 

 Sauropsidan tribe. But a connexion so low down seems unlikely, 

 now that we are acquainted with its tooth- structure ; since amongst 

 the multitude of numerous Sauropsidan species living and extinct, there 

 is not one which has inherited a tooth at all like that of the Ornitho- 

 rhynchus, but the teeth of every one such species is, as above stated, 

 formed upon a fundamentally different type ; this could hardly be 

 the case if the Ornithorhynchus tooth was derived from some archaic 

 form whence the Sauropsida, or any considerable section of them, were 

 also derived. But this tooth if not derived from a non-mammalian 

 animal, must either have been derived from some one amongst the 



* See Owen's 'Descriptive and Illustrated Catalogue of the Fossil Keptilia of 

 South Africa in the British Museum,' 1876, p. 15. 

 f ' Zool. Soc. Proc.,' 1880, p. 658. 



