Sauropoda were relatively short-lived, exist ing through part of the Triassic and the entire Jurassic, becoming apparently utterly extinct early in Comanchian time. What caused their extinction we do not know, probably a complex of causes of which the most important may have been a temporary restriction of their habi tat. In such relatively huge forms we have come to see evidences of racial senility, with which, as with old age in general, the hold upon life is precarious and a relatively small cause may have far-reaching and disastrous effects.
Ornithischia.— These dinosaurs have been called predentate, from the fact that the front of their mouth bore in the lower jaw a tooth less bone, the predentary, while except in one known instance the front of the upper jaw, the premaxillary, was toothless as well. But in the rear of both jaws were grinding teeth deeply implanted in open grooves and arising in suc cession from the depth of the groove outward later carnivores, and apparently could always be used to support the forward part of the body and for slow locomotion while feeding although swifter movement was effected by the hind limbs, with the tail for a counterpoise as in the Theropoda.
to replice the teeth consumed by wear. Al though in the earlier forms the number of teeth was not great, in the terminal member of one phylum, Trachodon (see Fig. 4), there were so many vertical rows of successional teeth that their total number equaled a half thousand for each half of each jaw, 2,000 more or less all told! The forward part of the mouth varied in form to suit the owner's habits: in Trachodon it was broadened like the beak of a duck; in others, such as Triceratops, it was more like that of a parrot or turtle; in all cases the bones must have been encased in horn or a leathery skin and served very well for the pre hension of the plant food which the highly effi cient dental battery reduced to a proper condi tion for digestion. This dental mechanism and the bird-like type of pelvis, although varying in detail, were the principal structures which these dinosaurs had in common; in other ways they varied enormously.
Ornithopoda.— These, the bird-footed dino saurs, were the unarmored bipeds which in their evolution paralleled the carnivores most closely. They had a four-toed foot armed with claws which in the larger forms became almost hoof-like. Their hands varied, but were gener ally four-toed and never possessed grasping claws. The fore limbs were much smaller than the hind, but never so absurdly so as in the The earlier types, such as Camptosassrus of the Jurassic and Comanchian, were small to medium in size, ranging from 7 to 17 feet.
Iguanodon of Belgium.and England was more ponderous, reaching 34 feet in length, and bore on the hand a spike-like thumb, the only weapon thus far recorded in the group. The later Cretaceous types show evidence, in their webbed feet and compressed swimming tail, of having been aquatic in habits. They are, perhaps, more completely known than any other dinosaurs, for not only are the bones and teeth preserved, but mummified carcasses, presenting with wonderful fidelity the skin, some muscles, and other details of structure usually lost in a fossil. The Orni thopoda were conservative except in their head, which, while generally simple, sometimes bore a helmet-like crest of curious form. Of such were Stephanosaurus and Corythosaurus from Alberta, Canada. Trachodon, although possess ing the most complex dentition, was plain of contour. The last named measured 23 feet in length, and was the terminal member of the race.
Stegosauria.— These were the armored dino saurs, whose body was covered with an increas ingly heavy cuirass and whose tail in some in stances possessed an investiture like a battle mace. The later forms, Ankylosaurus, must have been well-nigh immune to assault, and they mimicked curiously the armored glyptodons —warm-blooded mammals allied to the armadil los of to-day. The glyptodons had to meet the attack of the sabre-tooth tigers, while Ankylo saurus was pitted against the huge Tyranno saurus, but in each case passive resistance was apparently sufficient.
An earlier type, Jurassic to Comanchian, was Stegosaurus, the form which gives its name to the group. (See Fig. 5.) It represented an aber rant side line which died without further issue at the time of sauropod extinction, and like the latter group showed evidences of racial old age. Stegosaurus was a quadruped, short of fore limb, long of hind, with a highly arched back, powerful tail, and small head containing a defi cient dentition and absurdly diminutive brain. But the most striking peculiarity was its arma ment, which consisted of a double row of armor plates standing on edge, small on the neck, in creasingly large until the base of the tail was reached, and then once more diminishing. The end of the tail was armed with two or more pairs of long spikes which, together with the armor plates, were encased with horn. Stego saurs are found in Europe and the western United States.