ICHTHYOSAURS 469 



No Amphibia are certainly known from the Jurassic. 



The Reptiles have attained a higher and more diversified plane 

 of existence than in the Trias. Most of the Triassic genera and 

 one entire reptilian order, the Theromorphs, have become extinct, 

 but new and more advanced forms come in to take their places. 

 The Rhynchocephalians continue and the first of the true Lizards 

 (Lacertilia) appear. Turtles abound, having grown much more 

 numerous than in the Trias. The Ichthyosauria are a highly 

 characteristic Jurassic group ; for though they are found in both 

 the Trias and the Cretaceous, the Jura, and especially the Lias, is 

 the time of their principal expansion. Certain localities in the 

 Lias of England and Germany have yielded an incredible number 



FIG. 155. — Restoration of Ichthyosaurus quadriscissus. (E. Fraas.) 



of skeletons, and some of the specimens have preserved the im- 

 pressions of the outline of the body and limbs, showing recogniz- 

 ably the nature of the skin. These reptiles were entirely marine 

 in their habits and preyed upon fishes, and their limbs were con- 

 verted into swimming paddles ; there are several small dorsal fins 

 and a large tail-fin, the principal organ of propulsion (see Fig. 

 155). The muzzle is drawn out into an elongate slender snout, 

 armed with numerous sharp teeth, which were set in a continuous 

 groove, not in separate sockets. The eye is very large and pro- 

 tected by a number of bony plates, which are often preserved in 

 the fossil state. The neck is very short and hardly distinguished 

 from the porpoise-like body. The skin was smooth, having neither 

 horny scales nor bony scutes, which was of advantage in lessening 

 the friction of the water. In length, these reptiles sometimes ex- 



