ANCIENT SALAMANDERS gI 
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,! or “ roof-skulled.” 
This is clearly shown 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? 
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. 
* Greek—stegos, roof; cephalos, head. * Greek—condulos. 
