. La vie ancienne, histoire de la terre ; un projet exhaustif des principes et des faits de palaeontological la science. La paléontologie. r75. dents o^ ACJ-od7cs nobilis. {Notidanns de vrai requins) se produisent pour la première fois ; mais de loin le plus grand nombre d'remams appartenant à ce groupe sont encore les nageoires à rayons épineux et des dents de '', Cestracionts" ressemblant au vivant Requin Port-Jackson. Certaines de ces dents sont fait {Hybodus) ; mais d'autres sont arrondis, et sont adaptées pour broyer les coquillages. De ces derniers, les plus communs sont les dents d'Acrodus (fig. 175), dont l'obstacle à ceux que le

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. The ancient life-history of the earth; a comprehensive outline of the principles and leading facts of palæontological science. Paleontology. r75.—Tooth o^ Acj-od7cs nobilis. of true Sharks {Notidanns) 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 {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 colour, these teeth are commonly called "fossil leeches" by the quarrymen. The Amphibian group of the Labyrinthodo7its^ 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 /t7///^i'^i-^///7 or''Fish-Lizards" are exclusively Meso- zoic in their distribution but abounding especially in the former. Reptiles, of a fish-like form, with a hardly conspicuous neck (fig. 176), and probably possessing a simply smooth or are ranging from the Lias to the Chalk, They were huge. Fig. 176.—IchtJiyosa2ir7is coiuvnaiis 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 extr

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