98 • JURASSIC MAMMALS 



the family, is Plagiaulax. As it is not Triassic, the consideration 

 of its characters will be deferred until later. Microlcstes is a 

 Rhaetic genus, known from rocks in Germany and England ; 

 but it is entirely based upon molar teeth. M. mUiquus has a two- 

 rooted molar of an elongated form with a row of tubercles on either 

 side of a median groove, which traverses the long axis of the 

 tooth. To some extent the teeth of the ancient form resemble 

 those of Ornithorhynchus. Microlestes has been sometimes spoken 

 of as a Marsupial, but Mr. Tomes ^ has found that it does not 

 show one very universal character of the Marsupial teeth : 

 it has not those continuations of the dentinal tubes which 

 traverse the enamel in all Marsupials that have been examined 

 with the sole exception of the Wombat. 



The rarity of the remains of mammals in these earliest rocks 

 of the Secondary epoch has been accounted for in another way 

 from that which has been suggested above. It may be that the 

 group Mammalia was not evolved in Europe at all, and that the 

 stray remains which have been found in that continent represent 

 the fragmentary remnants of a few scattered immigrants which 

 heralded the later invasion of more luimerous genera diiring the 

 Jurassic period. 



The Mammals of the Jurassic Period. — Some of the Allo- 

 theria or Multituberculata described in the last section occur in 

 the rocks of this early part of the Secondary epoch. They are 

 doubtful in position, as already stated ; some of them indeed, 

 as for instance Tritylodoii and Dromatherium, are possibfy not 

 mammals at all, while the remainder probably belong to a non- 

 existent order of mammals. Along with these dubious creatures 

 are the fragmentary remains of small animals which are not 

 merely mammals, but in all probability definitely Marsupials. 

 It is true that here again we have little beyond lower jaws 

 and teeth to deal with ; so that there may be less certainty 

 in referring them to the Marsupials than appears to be the 

 opinion of the majority of Palaeontologists. 



Professor Osborn in fact considers that the Mesozoic mammals 

 consist of three groups: (1) The Multituberculata, including 

 the Bolodontidae, Stereognathidae, Plagiaulacidae, Polymasto- 

 dontidae, and possibly the Tritylodontidae (which, however, are 

 regarded by him and by others as more probably reptiles of the 



' Dental Anatomy, 5th ed. 1898, p. 304. 



