IV.] COMMUNITY OF EARLY FAUNA. 149 



many of them are very closely allied indeed, while some are 

 probably inseparable. Contemporary with these early marsupials 

 were members of the group known as Multituberculata, which are 

 probably more or less closely related to the existing monotremes, 

 or egg-laying mammals, and form with them the subclass Proto- 

 theria. An essential feature of these multituberculates is that 

 the molar teeth were divided by one or more grooves into longi- 

 tudinal ridges, covered with numerous blunt tubercles ; such 

 grooves being very generally two in number in the upper molars, 

 while the lower teeth have but a single one. Apparently in all 

 cases the extremities of the jaws were armed with a pair of chisel- 

 like incisor teeth, behind which in the upper jaw there may have 

 been a pair of smaller teeth. Very generally the last premolar 

 tooth, as in the English Purbeck genus Plagiaulax, was com- 

 pressed and trenchant in shape, with its upper edge regularly 



FIG. 28. RIGHT SIDE OF LOWER JAW OF Plagiaulax. Enlarged. 

 p. premolars, m. molars. 



convex, and its sides marked by oblique grooves; but in other 

 forms (Polymastodoti) this tooth was of a more tubercular type. 

 Without going into disputed questions, it may be stated that this 

 group was represented by closely allied forms in the Jurassic of 

 both Europe and North America; while it is also known, from 

 the evidence of a single genus (Tritylodon)^ to have extended its 

 range to South Africa ; this genus also occurring in the European 

 Trias, and thus affording another instance of the wide range of the 

 earlier faunas. 



Although in Europe the only known traces of mammalian life 

 during the succeeding Cretaceous period occur in the Wealden 

 beds (which are the immediate successors of the Jurassic Pur- 

 becks), in North America a well-developed fauna of polyprotodont 



