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.

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.

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.

Note the five digits on the hind feet, the quadrupedal gait, the enormous stretch of neck and tail, the small head aligned with the vertebral column.  Diplodocus was fully sixty-five feet long and must have weighed about twenty tons.  The thigh bones of the Sauropoda are the largest bones which ever grew.  That of a genus allied to the Diplodocus measures six feet and eight inches, and the total length of the animal must have been not far from eighty feet, the largest land animal known.

The Sauropoda became extinct when their haunts along the rivers and lakes of the western plains of Jurassic times were invaded by the Cretaceous interior sea.

The beaked dinosaurs(Predentata) were distinguished by a beak sheathed with horn carried in front of the tooth-set jaw, and used, we may imagine, in stripping the leaves and twigs of trees and shrubs.  We may notice only two of the most interesting types.