Because of the rigid structure of the jaws, united in front and incapable of any lateral movement posteriorly, quite as are the jaws of crocodiles, we are sure that prey of any considerable size could not have been swallowed whole. The crocodiles tear away portions of the flesh of their victims by quick, powerful jerks, and it is very probable that the flat-headed plesiosaurs tore their food apart in the same manner. In these kinds the teeth are much larger and more irregular in size than are those of the long-snouted plesiosaurs, and their use was certainly as much for tearing as for seizing. There are the same differences between the size of the head and the size of the teeth among the various plesiosaurs that there are among the modern crocodiles and gavials. While the crocodiles seize and destroy even larger prey, drowning and tearing their victims to pieces, the gavials are more exclusively fish-eating, for which their small, sharp, and more numerous teeth especially fit them. Their food, of small size, is swallowed entire, and they are comparatively harmless, so far as animals of considerable size are concerned. The long neck, the thickset body, and short, stout tail are not at all what we should expect to find in quick-swimming animals. We may therefore assume that the motions of the plesiosaurs through the water were more turtle-like than fish-like. The tail, even though provided with a terminal, fin-like dilatation, was of little use in the propulsion of the body, since the range of its movements was restricted; it possibly served in a measure as a steering organ, a rudder. The large, freely movable paddles must have been effective organs of locomotion, and this function accounts for the relatively large size of the posterior pair, and the firm union of the pelvis with the vertebral column through the sacrum. With the hind limbs used as oar-like organs, a firmer union with the skeleton was required than the soft yielding flesh would permit. At the same time this union was ligamentous only, not bony and unyielding, since the limbs were never used to support the body upon the ground; and it is of interest to observe that the ilia are directed, not upward and forward, but upward and backward to the sternum, precisely the position that would be expected with the force or thrust coming from behind, and not below the yielding ligaments. Were the tail longer and more powerful, the hind limbs would have been smaller and weaker, of use chiefly in equilibration, involving the loss of any connection with the vertebral column and the disappearance of the sacrum. It is of interest, finally, to observe that many of the slender-jawed plesiosaurs had a relatively short neck; they were doubtless more distinctively fish-eating in habit, and possessed greater speed. That the limbs of plesiosaurs were powerful propelling organs is also conclusively proved by their structure. Quite unlike all those animals whose locomotion in the water