The first of the extinct orders is that of the great Sea-lizards,[65] of which the now familiar _Ichthyosaurus_ and _Plesiosaurus_ of the English seas, to be seen in all museums and text-books, are the types (Figs. 142, 142_a_ and 142_b_). These were marine animals of large size, but not fishes or amphibians. They were true air-breathing reptiles, but with paddles for swimming instead of feet, and some of them with long flattened tails for steering and propulsion. They bore, in short, precisely the same relation to the other members of the class Reptilia which the Whales and Porpoises bear to the ordinary quadrupeds. Some of these animals are believed to have been fifty or sixty feet in length, thus rivalling the Whales, while others were of smaller dimensions, like the Porpoises and Dolphins. Some, like the _Ichthyosaurus_ and _Pliosaurus_ (Fig. 142_a_), were strongly built and powerful swimmers, and able to destroy the largest fishes, while others, like _Plesiosaurus_, had the body short and compact, the head small, and the neck long and flexible, and probably preyed on small animals near the borders of the waters. Catalogues of British fossils alone include about thirty species of Enaliosaurs, which haunted the coasts of Mesozoic Europe, a wonderful fact, when we consider the absence of these creatures from the modern seas, and the probability that only a fraction of the species are yet known to us. [Illustration: FIG. 142_a_.—Head of _Pliosaurus_. Jurassic. Much reduced.] [Illustration: FIG. 142_b_.—Paddle of _Plesiosaurus Oxoniensis_. Jurassic.—After Phillips. One-tenth natural size.] Another remarkable group is that to which Cope has given the name of _Pythonomorpha_, and which he regards as allied to the serpents, or as gigantic sea-serpents provided with swimming paddles, but which Owen considers more nearly connected with the lizards. In either case they constitute a group by themselves, remarkable not only on account of their anatomical affinities with animals so unlike them in general port, but also for their enormously extended length and formidable dentition (Fig. 143). Such animals as the _Mososaurus_ of Maestricht and _Clidastes_ of Western America may have exceeded in length the largest Ichthyosaurs and the most bulky of living Cetaceans, though their slender forms and numerous vertebræ remind one of the semi-fabulous sea-serpent, rather than of any known animal of our modern age. They were characteristic of the Later Mesozoic, more especially of the Cretaceous period, and must have been formidable enemies to the fishes of their time.