54 THE NOTOG/EIC REALM. [CHAP. 



of the opossums and bandicoots, in that they consist of an anterior 

 portion carrying three cusps in a triangle, and of a posterior 

 moiety or heel. Several more or less nearly related genera have 

 left their remains in the upper Jurassic rocks of the United States, 

 among which Dryolestes may be specially mentioned ; and it 

 appears that in North America the group survived till the succeed- 

 ing Cretaceous epoch. The especial interest attaching to these 

 marsupials is that they, and they alone, have molar teeth com- 

 parable in number, and to a certain extent in structure, with those 

 of the living Australian Myrmecobius ; and there can be but little 

 hesitation in regarding the latter as the specially modified descen- 

 dant of these ancient forms of mammalian life. It is, however, 

 important to notice that all the Jurassic types have four pairs of 

 lower incisor teeth, which are now retained by the opossums alone, 

 having been reduced in all the Australian polyprotodonts to three. 

 Although a very low type of mammal (Dromatherium) occurs 

 HOWAUS- i n t ^ ie Triassic rocks of North America, the fore- 

 traiia received going include all the leading extinct forms that can 

 be included among the marsupials. During the 

 Jurassic epoch the group seems to have been widely distributed 

 over Europe and North America ; it is known to have existed in 

 the latter area during the Cretaceous epoch, and it probably also 

 survived to that date in some part of the northern half of- the 

 Old World. After that, our knowledge is a blank till we meet 

 with the Oligocene opossums of Europe and North America, and 

 the Miocene Patagonian marsupials ; so that as regards the 

 Eocene epoch there is absolutely no record whatever of the group. 

 That Australia received its original fauna of polyprotodont 

 marsupials from the northward may be regarded as practically 

 certain ; and the question as regards the Notogseic realm accord- 

 ingly narrows itself to the approximate date of the immigration. 

 On this point Dr Wallace 1 writes that "it was probably far back 

 in the Secondary period that some portion of the Australian 

 region was in actual connection with the northern continent, and 

 became stocked with ancestral forms of marsupials ; but from that 

 time till now there seems to have been no further land-connection, 



1 Geographical Distribution of Animals, Vol. I. p. 465. 



