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 numerous genera during the Jurassic period. THE MAMMALS OF THE JURASSIC PERIOD.--Some of the Allotheria 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 _Tritylodon_ and _Dromatherium_, are possibly 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, Polymastodontidae, and possibly the Tritylodontidae (which, however, are regarded by him and by others as more probably reptiles of the {99} Theromorphous group). (2) The Triconodonta, which were Marsupials, though in all probability with a complete succession of teeth and with an allantoic placentation. This group will include the genera _Phascolotherium_ and _Amphilestes_, as well as _Triconodon_ and _Spalacotherium_. Finally we have (3) the Trituberculata (or Insectivora Primitiva) with the genera _Amphitherium_, _Peramus_, _Amblotherium_, _Stylacodon_, and _Dryolestes_.