170 ANIMAL LIFE PAST AND PRESENT. 



and in these alone among the whole class we find 

 in many cases that the jaws were armed with teetli 

 which were arranged in three groups corresponding 

 to the incisive, or front teeth, the tusks, or eye- 

 teeth, and the grinding teeth, of Mammals; so that 

 we have here another indication of the affinity of 

 this group with the latter. Another remarkable 

 peculiarity of the Anomodonts is that the perforation 

 in the lower end of the bone of the upper arm is 

 situated on the inner side or side corresponding to 

 the little finger as in Monotremes and many other 

 mammals (as the Cat) ; whereas in most other reptiles 

 this perforation is situated on the outer side or side 

 corresponding to the thumb. The upper arm-bone 

 itself of the Anomodonts is also wonderfully like that 

 of Monotremes. 



There are, indeed, many other features into the con- 

 sideration of which it would be quite out of place to 

 enter in this work which also lead to the same con- 

 clusion. We must, however, remember that Anomo- 

 donts agree with all other reptiles in having only one 

 condyle by which the skull articulates with the back- 

 bone ; and this difference may be a bar to regarding 

 them as the direct ancestral types of Mammals. The 

 extinct Amphibians known as Labyrinthodonts, in which 

 there were two condyles to the skull, are, however, as 

 stated in Chapter IV., so closely allied to the Anomo- 

 donts that it is in some instances difficult to draw a line 

 of distinction between the two ; and the conclusion has 

 therefore been reached that the Labyrinthodorit Amphi- 



