Turtles and tortoises are not found until the early Jurassic, when they already possessed the peculiar characteristics which set them off so sharply from other reptiles. They seem to have lived at first in shallow water and in swamps, and it is not until after the end of the Mesozoic that some of the order became adapted to life on the land. The largest of all known turtles, _Archelon_, whose home was the great interior Cretaceous sea, was fully a dozen feet in length and must have weighed at least two tons. The skull alone is a yard long. Lizards and snakes do not appear until after the close of the Mesozoic, although their ancestral lines may be followed back into the Cretaceous. We will now describe some of the highly specialized orders peculiar to the Mesozoic. =Land reptiles.= The _dinosaurs_ (terrible reptiles) are an extremely varied order which were masters of the land from the late Trias until the close of the Mesozoic era. Some were far larger than elephants, some were as small as cats; some walked on all fours, some were bipedal; some fed on the luxuriant tropical foliage, and others on the flesh of weaker reptiles. They may be classed in three divisions,--the _flesh-eating dinosaurs_, the _reptile-footed dinosaurs_, and the _beaked dinosaurs_,--the latter two divisions being herbivorous. The _flesh-eating dinosaurs_ are the oldest known division of the order, and their characteristics are shown in Figure 329. As a class, reptiles are egg layers (_oviparous_); but some of the flesh-eating dinosaurs are known to have been _viviparous_, i.e. to have brought forth their young alive. This group was the longest-lived of any of the three, beginning in the Trias and continuing to the close of the Mesozoic era. [Illustration: Fig. 329. Ceratosaurus] Contrast the small fore limbs, used only for grasping, with the powerful hind limbs on which the animal stalked about. Some of the species of this group seem to have been able to progress by leaping in kangaroo fashion. Notice the sharp claws, the ponderous tail, and the skull set at right angles with the spinal column. The limb bones are hollow. The ceratosaurs reached a length of some fifteen feet, and were not uncommon in Colorado and the western lands in Jurassic times. [Illustration: Fig. 330. Diplodocus] The _reptile-footed dinosaurs_ (Sauropoda) include some of the biggest brutes which ever trod the ground. One of the largest, whose remains are found entombed in the Jurassic rocks of Wyoming and Colorado, is shown in Figure 330.