Page:EB1911 - Volume 23.djvu/162

GENERAL CHARACTERS] elongation of the spines of their cervical and dorsal vertebrae (Dimetrodon, fig. 6). They seem to include various Triassic genera (e.g. Aetosaurus, Belodon), which may perhaps belong to the ancestral stock of the Dinosauria and Crocodilia. Other Triassic genera (Hyperodapedon, Rhynchosaurus) scarcely differ from Sphenodon, except in the dentition and in the absence of the pineal foramen in the skull. In the late Cretaceous and early Eocene periods one genus (Champsosaurus) was truly aquatic, with gavial-shaped head.

Order 6. —The dinosaurs are land reptiles which flourished on all the continents during the Jurassic and Cretaceous periods, in the interval between the decline of the Anomodontia and the dominance of the Mammalia. They first appeared as carnivorous reptiles in the Triassic period in Europe, India, S. Africa, and N. America, but afterwards comprised numerous massive herbivores in nearly all parts of the world except the Australian and New Zealand regions. The skeleton in the carnivorous dinosaurs, or Theropoda, is of very light construction, the vertebrae and limb bones being hollow, with thin, dense walls and often perfectly fitting joints. The fore limbs are small, and the hind limbs are adapted for running, jumping or hopping on the toes. The sabre-shaped cutting teeth are fixed in sockets, and all the claws are sharp. Auchisaurus and Hallopus, from the Trias of N. America, and Scleromochlus from the Elgin sandstones of Scotland, are comparatively small animals. Ceratosaurus and Megalosaurus, from the Jurassic of North America and western Europe respectively, must have attained a length of from 5 to 6 metres. Tyrannosaurus, from the Cretaceous of Montana, U.S.A., has a skull more than a metre in length. The herbivorous Dinosaurs of the suborder Ornithopoda resemble the Theropoda in general shape, but are heavier in build, with a pelvis constructed more nearly on the plan of that of a running bird. It has, indeed, been suggested that certain arboreal Dinosaurs of bipedal gait may have been the ancestors of the class Aves. The best-known Ornithopod is Iguanodon (fig. 7), from the Wealden of W. Europe, with species from 5 to 10 metres in length. Claosaurus, from the Cretaceous of N. America, is

nearly similar, and is represented by at least one complete skeleton in the Yale University Museum. There are also members of the same group with a heavy armour of bony plates and spines, sometimes termed Stegosauria. Stegosaurus itself occurs in the Upper Jurassic of Colorado, and Omosaurus, from the Kimmeridge and Oxford clays of England, is a nearly similar reptile. Polacanthus, from the Wealden of the Isle of Wight, has the hip-region armoured with a continuous bony shield. Triceratops (fig. 8) and its allies, from the Upper Cretaceous (Laramie) of western N. America, are the latest members of the group, with a bony frill over the neck, a pair of bony horn-cores above the eyes, and a median bony horn-core on the nose. The skull with the bony frill sometimes measures nearly two metres in length. Another suborder of herbivorous Dinosaurs, that of Sauropoda, comprises the largest known land animals of any age, some measuring from 17 to 25 metres in total length. They have a small head, long neck, and long tail, and must have been quadrupedal in gait. Their teeth are adapted for feeding on succulent water weeds, perhaps with an admixture of small animals 1iving among these; and their vertebrae are of very light construction, while the ribs are raised high on the neural arches to increase the size of the body cavity, perhaps for unusually large lungs or air sacs. Their massive limbs have five toes, of which the three inner alone bear outwardly curved claws. Diplodocus and Brontosaurus, from the Jurassic of Wyoming and Colorado, U.S.A., are the best-known genera. Atlantosaurus, from the same formation, is usually noteworthy for size. Cetiosaurus, from the Jurassic of England, is also known by large parts of the skeleton in the British Museum and the Oxford Museum, indicating species nearly 20 metres in length.

Order 7. —Typical crocodiles can be traced downwards to the Lower Lias at the base of the Jurassic