102 REPTILES 



or internal, beyond the webbing of the feet in crocodiles and 

 certain peculiarities in connection with the breathing apparatus, 

 has been evolved to adapt them to an aquatic life. On the 

 other hand, there are several distinct groups of reptiles in which 

 some or all of the species have been profoundly modified in 

 accordance with the needs of a pelagic mode of existence. Such 

 are the sea-crocodiles, Geosaurus, Dacosaurus, and Metrio- 

 rhynchus, constituting the family Geosaurida? of the order 

 Crocodilia; the ichthyosaurs or fish-lizards forming the order 

 Ichthyopterygia ; the plesiosaurs and pliosaurs, representing the 

 order Sauropterygia ; the mosasaurs, or " sea-serpents " [Mosa- 

 sauruSy etc.) which form a separate suborder of the Squamata ; 

 and the dolichosaurs (Dolichosauria), which constitute a second 

 subordinal group of the latter order. The turtles, again, repre- 

 sent a specialised group of the order Chelonia modified for an 

 aquatic life. In the latter case the modification has not been 

 carried to the same extent as in the other groups, for these 

 reptiles habitually resort to the shore at certain seasons, whereas 

 some of the former were as purely pelagic as are whales and por- 

 poises at the present day, Ophthalmosaurus being the most so 

 among the ichthyosaurs. It is curious that all the above-men- 

 tioned groups of pelagic reptiles are extinct at the present day, 

 their place (unless the " sea-serpent " be a reality) having been 

 taken by the Cetacea, which have ousted them in the struggle 

 for existence. Further, each of these extinct groups has been 

 (like the Cetacea) independently evolved from terrestrial forms 

 although it is not in every instance possible to point to the 

 ancestral type. Had they been derived from more primitive 

 groups of marine vertebrates, such as fishes, it is evident that, 

 like the latter, they would have breathed the air dissolved in 

 water by means of gills, instead of being compelled to rise 

 (like whales) at intervals to empty and refill their lungs with 

 atmospheric air. In every case one pair of limbs has been 

 converted into paddles, or flippers, with digits united by a 

 common integument ; but the structure of the skeleton of these 

 paddles is subject to great variation in the different groups ; 

 this being alone sufficient to demonstrate the independent origin 

 of the latter. Moreover, apart from the turtles, a more or less 

 whale-like or eel-like type of body has been assumed by the 

 members of these pelagic groups. 



