130 THE OLDEST MAMMALS. 



to have totally disappeared. At a still later date a single 

 tooth of the Plagiaulax type, as well as one allied to 

 Bolodon, have been described from the English Wealden, 

 indicating that at that epoch the Pur beck mammals still 

 survived in Europe, and leading to the hope that future 

 researches will yield us further evidence of the European 

 mammalian fauna of the Wealden. 



The present state of our knowledge, therefore, shows 

 that from nearly the lowest beds of the Secondary period 

 till the close of that vast epoch, there existed a numerous 

 fauna of small mammals distributed over a large portion 

 of the globe, and displaying a remarkable persistence of 

 nearly similar type. It is further evident that such of 

 these mammals as exhibit a carnivorous type of dentition 

 appear to be allied to the more primitive of the existing 

 marsupials, although some of them may be more nearly 

 related to the almost equally low insectivores. On the 

 other hand, those which exhibit what appears to be an 

 herbivorous modification of dental structure, if they are 

 related to any living forms, appear to have an affinity with 

 the modern egg-laying mammals of Australia. Now the 

 latter, together with the marsupials and insectivores, being 

 the lowest representatives of mammalian life at present 

 existing, are precisely such mammals as we should natu- 

 rally have expected to have been foreshadowed by more 

 or less nearly allied forms in the Secondary rocks ; and, 

 therefore, in this respect, theoretical palaeontology is, so 

 far as our present knowledge goes, precisely in accord 

 with actual facts. 



That the few Triassic mammals at present known were 

 the earliest representatives of the class cannot, however, 

 be admitted for a moment, and we must accordingly look 

 either to the lower Triassic rocks, or to those of the 

 underlying Permian (forming the top of the Palaeozoic 

 series), for the discovery of such primitive types. Should 

 such ever be discovered, it is to be confidently expected 

 that they will exhibit such a combination of characters 

 common to mammals, and certain extinct reptiles and 

 amphibians, that it will be very hard to say under what 

 class they will have to be ranked. 



