EGG-LAYING MAMMALS, OR MONOTREMES. 171 



bians, Anomodont Reptiles, and Monotreme Mammals 

 have all taken origin from one common amphibian 

 stock ; the Anomodonts retaining marked evidences of 

 their kinship to the Mammals which have been totally 

 lost in all other reptiles. 



Such, then, is very briefly the chief deductions 

 which can be drawn from the life-history and structure 

 of the Duck-bill and Echidna as to their affinities with 

 other groups of animals; and it is curious to reflect 

 that but for the preservation in a remote part of the 

 world of these two representatives of a group which 

 must in all probability have been once abundant, we 

 should have been utterly unable to trace this wonderful 

 relationship of the mammals to the earlier types of 

 reptiles and amphibians. 



At present we have, indeed, no absolutely conclusive 

 evidence of the former existence of mammals allied to 

 the living Monotremes, although there is a certain 

 amount of evidence tending in this direction. Thus 

 in the Secondary rocks, dating from the Trias to the 

 Chalk, and also in the lowest beds of the overlying 

 Tertiary, we meet with a number of small mammals 

 characterised by a very peculiar type of grinding- teeth. 

 The crowns of these teeth (Fig. 5), consist of two or 

 three longitudinal ridges carrying distinct cusps, and 

 separated from one another by deep grooves, which 

 may be either one or two in number. This type of 

 tooth, which is quite unlike that found in any of the 

 higher mammals, presents a certain approximation 

 to the deciduous teeth of the Duck-bill ; so that it is 



