174 SKETCHES OF (JJREATION. 



each other. The latter circumstance led to the opinion 

 that the posterior limbs of the reptile were much stouter 

 than the anterior, as in the kangaroo and frog. When the 

 bones of these animals were brought to light, geologists had 

 the opportunity to certify themselves that these problem 

 atical hand-prints were impressed by reptilian instead of 

 mammalian quadrupeds ; and that while the weight of 

 characters allied them to true reptiles, they nevertheless 

 possessed strong analogies with Batrachians, and probably 

 simulated the form and habits of the frog though in truth 

 we should say that the frog was subsequently fashioned in 

 the similitude of a Labyrinthodont. The head was helmet- 

 ed by a pair of broad, bony plates, through which were 

 openings for the eyes ; and some parts of the body were 

 covered, especially in the later ages, by a similar armor. 

 The striking characteristic of these ancient reptiles, from 

 which they receive their name, is seen when a very thin 

 transverse section or slice of one of the teeth is viewed un 

 der the microscope. The external coating of the tooth, 

 called cement, is folded inward in folds which reach to the 

 central cavity, and in their course are inflected into a laby 

 rinth of subordinate lateral folds. Some of these frog-like 

 quadrupeds seem to have attained the size of an ox. It is 

 likely that they were the representatives of the class of 

 Batrachians in those early periods, as no other Batrachia 

 are known in the Trias ; and those before alluded to from 

 the coal-measures are known likewise to have possessed 

 the peculiar cephalic plates of the Labyrinthodonts. 



The Triassic Age witnessed also the advent of multi 

 tudes of marine saurians of the family of Ichthyosaurs, hav 

 ing enormous cavities in their craniums for the lodgment 

 of the eyes. This type of reptiles is restricted to this sin 

 gle age of the world, f lere also crawled reptiles resem 

 bling gigantic lizards, semi-aquatic or purely terrestrial in 



