H54 



THE POUCHED MAMMALS, OR MARSUPIALS 



EXTINCT MARSUPIALS 



LOWER JAW OF TRICONODON. 



(Three times natural size.) 

 (After Marsh.) 



From their low degree of organization it would be expected that Marsupials 

 are some of the oldest of Mammals, and this expectation is borne out by the facts. 

 So far as can be determined, no Placental Mammals are known to have existed be- 

 fore the Tertiary period, that is to say, in the rocks lying below the London clay. 

 The cretaceous rocks of North America, and the under-lying Jurassic or Oolitic 

 rocks both of this continent and of Europe have, however, yielded a number of re- 

 mains of small Mammals which may be pretty confidently assigned to the Polypro- 



todont section of the present 

 order. In one form, known 

 as Triconodon, the molar teeth, 

 of which there were four in 

 the fully adult state, although 

 only three are shown in the 

 ~3 lower jaw here figured, are 



characterized by carrying three 

 compressed cones arranged in 

 a line one before the, other; 

 while the premolars, three in number, were simpler. The groove ( g ) seen on the 

 inner side of the lower jaw, corresponds to one found in the banded ant-eater 

 and a few other living Marsupials, but un- 

 known in any other Mammals. In a second 

 type, as represented by Amphilestes from 

 the Stonesfield beds near Oxford, the cheek- 

 teeth were much more numerous, and the 

 molars less unlike the premolars. In the 

 molar teeth the front and hind-cones were rel- 

 atively smaller in proportion to the middle 

 one than is the case in Triconodon; and in the number and form of these teeth 

 this early Mammal comes exceedingly close to the living Australian banded ant-eater. 



A third type is represented by 

 jaws from the Purbeck rocks of 

 Dorsetshire, known as Amblothe- 

 rium, closely allied to which is 

 the jaw from America represented 

 in our third figure. Here the 

 front and hind-cones have be- 

 come twisted round to the inner 

 side of the main cone, so that 

 the crown of each molar forms a 

 triangle, as in the living bandi- 



LOWER JAW OF A MARSUPIAL ( Amphi- 



lestes] FROM THE STONESFIELD SLATE. 



(Twice natural size.) 



LOWER JAW OF AN AMERICAN JURASSIC MAMMAL. 



( Twice natural size.) 



(After Marsh.) 



coots and opossums. The number of the cheek-teeth is, however, much greater 

 than in the latter, and thus indicates relationship with the banded ant-eater. 



