Perhaps the best way in which we can gain a clear idea of the general characters of a long-necked sea-lizard, as we may call our Plesiosaurus, is by comparing it with the fish-lizard, described in the last chapter. Its long neck and small head are the most conspicuous features. Then we notice the short tail. But if we compare the paddles of these two extinct forms of life, we notice at once certain important differences. In the fish-lizard the bone of the arm was very short, while all the bones of the fore-arm and fingers were modified into little many-sided bodies, and so articulated together as to make the whole limb, or paddle, a solid yet flexible structure. In the long-necked sea-lizard, however, we find a long arm-bone with a club-like shape; and the two bones of the fore-arm are seen to be longer than in the fish-lizard. But a still greater difference shows itself in the bones of the finger, as we look at a fossilised skeleton (or a drawing of one); for the fingers are long and slender, like those of ordinary reptiles. There are only five fingers, and each finger is quite distinct from the others. This is the reason why the Plesiosaur was considered to depart less from the type of an ordinary reptile, and so received its name. Other remarkable differences present themselves in the shoulders and haunches, but these need not be considered here. The species shown in Fig. 8 had rather a large head. It is obvious that such a long slender neck as these creatures had could not have supported a large head, like that of the fish-lizard. Consequently, we find a striking contrast in the skulls of the two forms. That of the Plesiosaur was short and stout, and therefore such as could easily be supported, as well as rapidly moved about by the long slender neck. Thus we find another simple illustration of the "law of correlation," alluded to on p. 6. The teeth were set in distinct sockets, as they are in crocodiles, to which animals there are also points of resemblance, in the backbone, ribs, and skull. Fig. 7 shows three different types of lower jaws of Plesiosaurs. The one marked C belongs to Plesiosaurus dolichodeirus, the species represented in our plate. There were no bony plates in the eye. Professor Owen thinks that they were long-lived. The skin was probably smooth, like that of a porpoise. [Illustration: Plate III. PTERODACTYLS. LONG-NECKED SEA-LIZARD. CUTTLE-FISH OR BELEMNITE. _Plesiosaurus dolichodeirus._ Length 22 feet.] [Illustration: Fig. 7.--Mandibles of Fish-lizards. A, _Peloneustes philarchus_ (Seeley); from the Oxford Clay. B, _Thaumatosaurus indicus_ (Lydekker); Upper Jurassic of India. C, _Plesiosaurus aolichodirus_ (Conybeare); from the Lower Lias, Lyme Regis.]