The skull of the long-necked plesiosaurs is surprisingly small in comparison with the remainder of the skeleton, often very snake-like in shape, though very un-snake-like in structure. The short-necked plesiosaurs had often a relatively larger skull, in _Pliosaurus_, for instance, more than five feet long, sometimes rather broad and short, sometimes remarkably long and slender. The external nostrils were situated far back, very near the eyes, and were very small. The eyes, of considerable size, though by no means so large as those of the ichthyosaurs, were directed laterally, and were provided with a ring of bony sclerotic plates—rather small and weak ones, however. The quadrate bones—bones peculiar to the reptiles and birds—to which the lower jaws are articulated, are, as in the ichthyosaurs and crocodiles, rigidly fixed and immovable. The lower jaws, always rather slender, are firmly united in front, sometimes for a long distance, as in the modern gavials. The teeth of the broad-headed plesiosaurs are long, slender, pointed, and recurved, of a murderously cruel shape; they are deeply implanted in sockets, and number from twenty to thirty on each jaw above and below. There are no teeth on the bones of the palate, such as the mosasaurs possessed. The slender-jawed, gavial-like plesiosaurs have more numerous, but smaller teeth. The surface of the skull on each side behind, for the attachment of the muscles closing the mandibles, is of great extent; in some this surface is increased by a high, thin crest in the middle, as in strongly carnivorous animals, all of which give conclusive evidence of the powerful muscles used in biting and seizing. There is but one temporal opening on each side, as in the ichthyosaurs and the mosasaurs, whereas the crocodiles, thalattosaurs, phytosaurs, etc., have two. The brain cavity of all plesiosaurs is small, though the cavities of the internal ears, the semicircular canals at least, are large. The semicircular canals in vertebrates have little or nothing to do with the function of hearing; they serve rather for equilibration, for the co-ordination of muscular movement; possibly we may infer from their large size in the plesiosaurs that they were not at all clumsy in their movements. There is a large opening for the pineal body, the so-called eye in the roof of the brain cavity, though its possession does not necessarily imply the possession of a functional organ. [Illustration: FIG. 41.—Skull of _Trinacromerum_ from the side: _ang_, angular; _d_, dentary; _pm_, premaxilla; _po_, postorbital; _j_, jugal; _sur_, surangular.]