No known reptiles lay their eggs in the water. Perhaps there is some reason why the eggs of reptiles and birds, so different from those of fishes and amphibians, cannot hatch in water; and there is no good reason for supposing that the mosasaurs were exceptions to this rule. Unless carefully hidden or protected by the parent, the eggs or very young of the mosasaurs would have been subject to many and grave dangers. Fish eggs are usually small and produced in great numbers, thousands often being extruded from a single female. Among so many there is a greater probability that at least two will hatch and survive to maturity, reproducing their kind. It is unreasonable to suppose that the lizards of the past were more prolific of eggs than are their relatives now living; nor is it possible that their eggs could have been as small as are those of most fishes. Modern lizards seldom lay more than twenty-five or thirty eggs at a time; even the turtles, with their greater vicissitudes, seldom produce more than one hundred. The eggs of the mosasaurs were certainly large and few in number, and the young animals must have begun breathing air immediately after escaping from the shells. If the mosasaurs were oviparous they must have laid their eggs upon the shores and beaches, as do the sea-turtles and the Crocodilia. Nor is it at all probable that the female mosasaurs gave even that protection to their eggs or young that the crocodiles and turtles give. The young mosasaurs, perhaps reaching a foot in length, must have been left entirely to their own devices and their own fate at the very earliest stages of their independent careers. The waters in which the mosasaurs abounded swarmed with many kinds of predaceous fishes, to say nothing of the hordes of their own kinds, all carnivorous in the highest degree, to all of which the tender saurians must have been choice food. Possibly the shallow waters of the bays and estuaries may have afforded protection to the newly hatched reptiles. It would seem probable that the female mosasaurs went up the rivers for a shorter or longer distance to lay their eggs or give birth to their young, and that the young reptiles remained in such relatively protected places until of a sufficient size to cope with the fierce enemies of the open seas. We know practically nothing of the inhabitants of the lakes and rivers during all the time in which the mosasaurs existed; and this perhaps is the real reason why we have never yet found a specimen of a young mosasaur. [Illustration: FIG. 79.—Head of _Tylosaurus_.]