282 POUCHED MAMMALS. 
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 JT'riconodon, 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 Amphi- 
lestes from the Stonesfield beds near Oxford, 
the cheek-teeth were much more numerous, = ; 
: LOWER JAW OF A MARSUPIAL (Amphilestes) FROM 
and the molars less unlike the premolars. THE STONESFIELD ‘SLATE (twice nat. size). 
In the molar teeth the front and hind- 
cones were relatively 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 anteater. A 
third type is represented by 
jaws from the Purbeck rocks of 
Dorsetshire, known as Amblo- 
theriwm, 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 TRICONODON (3 times nat. size).—After Marsh. 

LOWER JAW OF AN AMERICAN JURASSIC MAMMAL 
(twice nat. size).—After Marsh. 
