In the class of the _Reptiles_, the disappearance of the characteristic Mesozoic types is as marked a phenomenon as the introduction of new forms. The Ichthyosaurs, the Plesiosaurs, the Pterosaurs, and the Mosasaurs of the Mesozoic, find no representatives in the Eocene Tertiary; and the same is true of the Deinosaurs, if we except a few remains from the doubtfully-situated "Lignitic formation" of the United States, On the other hand, all the modern orders of Reptiles are known to have existed during the Eocene period. The _Chelonians_ are represented by true marine Turtles, by "Terrapins" (_Emydidoe_), and by "Soft Tortoises" (_Trionycidoe_). The order of the Snakes and Serpents (_Ophidia_) makes its appearance here, for the first time under several forms--all of which, however, are referable to the non-venomous group of the "Constricting Serpents" (_Boidoe_). The oldest of these is the _Paloeophis toliapicus_ of the London Clay of Sheppey, first made known to science by the researches of Professor Owen. The nearly-allied _Paloeophis typhoeus_ of the Eocene beds of Bracklesham appears to have been a Boa-constrictor-like Snake of about twenty feet in length. Similar Python-like Snakes (_Paloeophis, Dinophis_, &c.) have been described from the Eocene deposits of the United States. True Lizards (_Lacertilians_) are found in some abundance in the Eocene deposits,--some being small terrestrial forms, like the common European lizards of the present day; whilst others equal or exceed the living Monitors in size. Lastly, the modern order of the _Crocodilia_ is largely represented in Eocene times, by species belonging to all the existing genera, together with others referable to extinct types. As pointed out by Owen, it is an interesting fact that in the Eocene rocks of the south-west of England, there occur fossil remains of all the three living types of Crocodilians--namely, the Gavials, the true Crocodiles, and the Alligators (fig. 226)--though at the present day these forms are all geographically restricted in their range, and are never associated together. [Illustration: Fig. 226.--Upper jaw of Alligator. Eocene Tertiary, Isle of Wight.]