282 



POUCHED MAMMALS. 



M^m^ 



LOWER JAW 



-After Marsh. 



existed before 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 that continent and of Europe have, 

 however, yielded a number of remains of small mammals which may be pretty 



confidently assigned to the 

 Polyprotodont 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 lower jaw here 

 figured, are characterised 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 

 anteater and a few other living ^Marsupials, 

 but unknown in any other mammals. In 

 a second type, as represented by Arnphi- 

 lestesirom the Stonesfield 1 r 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 relatively smaller in proportion to the middle one than is the case in 

 Trieonodon ; and in the number and form of these teeth this early mammal comes 



exceedingly close to the living 

 Australian banded anteater. A 

 third type is represented by 

 jaws from the Purbeck rocks of 

 Dorsetshire, known as Amblo- 

 tht ;•/(' in. closely allied to which 

 is the jaw from America repre- 

 sented in our third figure. Here 

 the front and hind-cones have 

 become twisted round to the 

 inner side of the main cone, so 

 that the crown of each molar forms a triangle, as in the living bandicoots and 

 opossums The number of the cheek-teeth is, however, much greater than in the 

 latter, and thus indicates relationship with the banded anteater. 



LOWER JAW OF A HABST. 



the stoxesfield slate (twice nat. size). 



LOWER JAW OF AN AMERICAN JURASSIC .MAMMAL 



(twice nat. size). — After Marsh. 



