242 



HISTORICAL PALAEONTOLOGY. 



Tooth o? Acrodus nobilis. 



of true Sharks [Notidamis) occur for the first time ; but by far 

 the greater number of remams 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 

 iyHybodus) ; 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, 

 and their dark colour, these 

 teeth are commonly called 

 " fossil leeches " by the quarrymen. 



The Amphibian group of the Labyj^tnthodonts, 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 

 Plesiosaiirus. 



The Ichthyosauri Q)X '■'' Y\'i\i-\Jvz2.TA^'" 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 



Fig. 176. — Ickthyosaitrtis covivnaiis. Lias. 



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 



