1272 CLASS MAMMALIA. 



and in many genera the palatal region of the cranium contains 

 unossified vacuities of considerable size. 



Order II. Marsupialia. — The whole of the known Metatheria 

 are included in the single order Marsupialia, and it is not at present 

 necessary to give ordinal characters as distinct from those of the 

 subclass. At the present day this order is restricted to America 

 and the Australian region ; the greater number of forms occurring 

 in the latter area, while only the Didelphyidce. are found in the former. 

 In earlier epochs, however, this order was much more widely dis- 

 tributed, and it apparently contains some of the earliest known 

 representatives of the entire class. 



Marsupials form in some respects the intermediate stage between 

 the Prototheria and Eutheria, and it is probable that there will be 

 eventually found a complete transition from the Polyprotodont sub- 

 order of this group to unknown Prototherian Mammals with the 

 same general type of dentition — possibly more or less closely allied 

 to the undermentioned Triassic family Dromatheriidce. 



Suborder i. Polyprotodontia. — In existing Polyprotodont 

 Marsupials, which occur both in America and the Australian region, 

 there are never fewer than three lower and four upper incisors, and 

 there may be five upper and four lower teeth of this series. These 

 teeth are small and subequal, and are followed by a larger canine 

 (fig. 1 148). Normally there are three premolars, corresponding to 

 the first, third, and fourth of the typical series, but the fourth may 

 be absent, Dasyurus and Sarcophilus (fig. 1148). The number of 

 true molars is generally four, but in Myrmecobius these teeth are 



increased to - ; and their crowns are nearly always characterised by 



carrying a number of small, sharply pointed cusps. In no species 

 is there a fourth premolar of the elongated secant form characteristic 

 of many Diprotodonts. Several of the Mesozoic forms included in this 

 group agree very closely with the recent ones, but many of them have 

 four premolars, and in some there are as many as seven true molars. 

 Family Dromatheriidce. — This family, typically represented by 

 the genus Dromatherium (fig. 1140), of the reputed Trias of North 



America, is provisionally placed 

 here, since it may be related to 

 some of the members of the next 

 family, although Professor Osborn 

 makes it the type of a distinct 



Fig. 1140.— Inner view of the left ramus of j ,i t-» ±. j j 



the mandible of Dromatherium sylvestre ; Order the Protodonta and 



Emmons!) 1 "* 5 ° f Nonh Carolina ' (After even suggests that it may be 



Reptilian. If not Marsupials, 

 these early Mammals may prove to be representatives of a group of 

 Prototheria from which the Polyprotodont Marsupials have origin- 



