the internal structure of the teeth we have already spoken ; but 

 although the same complexity of folding does not apply to some 

 of the smaller forms, the name by which they are known is very 

 appropriate to the group as a whole. In Plate IX. the reader 

 will find a restoration of a typical labyrinthodont the Mastodon- 

 saurus, of which the skull is shown in Fig. 16. There was a 

 marked difference in size between the hind limbs and fore limbs, 

 as shown in the above Plate. There are five digits to each limb, 

 and no claws. 



Turning to their skulls, there is one feature that is quite 

 peculiar to the group and rarely seen in reptiles, living or 

 extinct, viz. that the whole of the upper surface of the skull 

 behind the eye-holes is covered in by a complete roof of bone. 

 Hence some authorities call them Stegocephali, 1 or " roof-skulled." 

 This is clearly sM^ ,in Fig. 16, representing the huge skull of 

 Mastodonsaurus. Some fishes have skulls thus roofed over. But 

 the skull of a lizard or a crocodile is very different, showing a 

 long open channel behind each orbit, and another one lower 

 down at the side of the head.. Another important character in 

 the labyrinthodont skull is the presence of two condyles, instead 

 of only one, as in all true reptiles and birds. The word condyle 2 

 signifies a knuckle, and is applied chiefly to the surface or 

 surfaces by which the skull joins on to, or articulates with, the 

 first vertebra of the neck. All the mammalia have skulls 

 articulating by two condyles a fact which would seem to imply 

 that mammals branch off from the amphibia, and not from reptiles. 

 Again, the bones of which the skull is composed are nearly 

 always covered with a network of grooves, or " canals," doubtless 

 intended for the attachment of hard, horny, and bony scales or 

 scutes. 



1 Greek steyos, roof; cephalos, head. 2 Greek condulos. 



