THE JURASSIC PERIOD. 



249 



was a large ink-sac; and the mouth was armed with horny 

 mandibles resembling in shape the beak of a parrot. 



Coming next to the Vertebrates, we find that the Jurassic 

 Fishes are still represented by Ganoids and Placoids. The 

 Ganoids, however, unlike the old forms, now for the most 

 part possess nearly or quite symmetrical (" homocercal") tails. 

 A characteristic genus is Tetragonolepis (fig. 174), with its 

 deep, compressed body, its rhomboidal, closely-fitting scales, 

 and its single long dorsal fin. Amongst the Placoids the teeth 

 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 striae proceed- 

 ing from a central longi- 

 tudinal line. From their 

 general form and striation, 

 and their dark color, 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 



Fig. 175. Tooth of Acrodua nobilis. Lias. 



Fig. 176. Ichthyosaurus commwnis. Lias. 



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. 



