242 HISTORICAL PALEONTOLOGY. 



of true Sharks (Notidanus) occur for the first time ; but by far 

 the greater number of remains referable to this group are still 

 the fin-spines and teeth of " Cestracionts," resembling the 

 living Port-Jackson Shark. Some of these teeth are pointed 

 (Hybodus) ; but others are rounded, and are adapted for crush- 

 ing shell-fish. Of these latter, the commonest are the teeth of 

 Acrodus (fig. 175), of which the hinder ones are of an elon- 

 gated form, with a rounded 

 surface, covered with fine 

 transverse stride proceed- 

 ing from a central longi- 

 tudinal line. From their 

 general form and striation, 

 Fig. i 75 .-Tooth tf Acrodus nobiiis. Lias. and their dark colour, these 



teeth are commonly called 

 " fossil leeches " by the quarrymen. 



The Amphibian group of the Labyrinthodonts, which was so 

 extensively developed in the Trias, appears to have become 

 extinct, no representative of the order having hitherto been 

 detected in rocks of Jurassic age. 



Much more important than the Fishes of the Jurassic series 

 are the Reptiles, which are both very numerous, and belong to 

 a great variety of types, some of these being very extraordinary 

 in their anatomical structure. The predominant group is that 

 of the " Enaliosaurs " or " Sea-lizards," divided into two great 

 orders, represented respectively by the Ichthyosaurus and the 

 Plesiosaurus. 



The Ichthyosauri or " Fish-Lizards " are exclusively Meso- 

 zoic in their distribution, ranging from the Lias to the Chalk, 

 but abounding especially in the former. They were huge 

 Reptiles, of a fish-like form, with a hardly conspicuous neck 

 (fig. 176), and probably possessing a simply smooth or 



wrinkled skin, since no traces of scales or bony integumentary 

 plates have ever been discovered. The tail was long, and 

 was probably furnished at its extremity with a powerful ex- 

 pansion of the skin, constituting a tail-fin similar to that pos- 

 sessed by the Whales. The limbs are also like those of Whales 



