Diplodocus longus, of the Upper Jurassic of Colorado and Wyoming, is almost completely known. More than 40 feet long, it had a head in its general outlines not unlike that of a horse, the skull being about two feet long. The outer nasal openings are confluent, elongated, and lie far back on the top of the skull. There is a pair of large antorbital, and a pair of smaller lacrymal fossae. The teeth, long and slender, are restricted to the anterior portion of the mouth, with many successors, which, decreasing in size, lie on the inner or lingual side of the functional tooth, like the cartridges in a repeating rifle. The functional teeth themselves are implanted in sockets. The generic name refers to the peculiar chevron-bones, each half of which diverges into an anterior and a posterior branch.
It is difficult to understand how these huge, long-necked Sauropoda lived and moved about. The long neck suggests at first sight predacious habits, but the teeth, rather feeble in Diplodocus, and distinctly of the plant-cutting type in other genera, put this out of the question. The high position of the unpaired nasal opening, and the shortened nasal bones of Diplodocus, are features indicative of aquatic habits, but the short-toed, plantigrade limbs are absolutely adapted to terrestrial life, and we cannot well assume that such enormous brutes as Atlantosaurus could possibly have ventured into swampy ground.
Order II. THEROPODA.
Pubes simple, with symphysis. Premaxillae with teeth. Digitigrade. Carnivorous.
The teeth are pointed, recurved, laterally compressed and serrated. The nasal openings are large, lateral, and nearly terminal. The vertebrae and the large bones of the limbs are hollow. The fore-limbs are considerably shorter than the hind-limbs, which are distinctly digitigrade, many of the species having a pronouncedly upright gait. The proximal tarsalia show a tendency to fuse with the tibia, and the astragalus has sometimes an ascending process, by which the fusion with the tibia is strengthened. The first and fifth metatarsals are often reduced, while the three middle bones are elongated and sometimes even fused with each other, so that the whole foot assumes a striking resemblance to that of birds. The terminal phalanges are protected by curved claws. Owing to the shortness of the fore-limbs, and the often considerable length of the hind-limbs, which are strongly bent at the knee and the ankle-joint, these animals must have progressed somewhat like clumsy kangaroos.
The Theropoda, of which a great number of genera are now known, from the size of a slender cat to that of an elephant, lived from the Upper Trias to the Upper Oolite, both in Europe and in North America.
Brontozoum giganteum, one of the oldest forms, is known from its foot-spoors only, which, together with other three-toed spoors in the sandstone of the Connecticut valley, were originally described and figured by Hitchcock as Ornithichnites (ἴχνος = track, or spoor). Some of these imprints are more than a foot in length, the right and left spoors following alternately at a distance of from four to six feet. In some cases the long trailing tail has left a furrow behind, and the large tracks are accompanied or crossed by much smaller, and even by quite tiny tracks, otherwise similar, and undoubtedly made by the young.
Anchisaurus, from the same locality, was still Sauropodous, in so far as the metatarsals are still free, with two, three, four, and five phalanges on the first four toes, but the fifth metatarsal is reduced, carrying a vestige of only one phalanx, and the proximal tarsal bones are fused with the tibia and fibula respectively. Total length some seven feet, of which about four belong to the tail.
Zanclodon, from the Keuper of Würtemberg, about ten feet long, with pentadactyle hands and feet. Ischia stronger than the pubic bones, which are distally much broadened. The femur is nearly three feet long, and possesses a fourth trochanter. The astragalus has an ascending process, and is fused with the tibia. The toes are short, strong, and clawed. The shoulder-girdle and fore-limb are strong, the latter well adapted to grasping. The teeth are much compressed laterally, with sharp, finely serrated edges. Several allied genera have been described from the Upper Trias of France and England: others from corresponding strata of India and South Africa.
