Dinosauria

Dinosauria, an extinct order or sub-class of reptiles, only known at present from Secondary rocks. They are very varied in size and form, some terrestrial, others aquatic or amphibious; some carnivorous, but the majority herbivorous. The vertebree are flat or slightly concave, those of the neck numerous. The hind limbs are longer and larger, and with the bones of the pelvis present striking resemblances to those of the ostrich-like birds. Some genera at least walked erect, their powerful tails assisting in balancing their bodies. Some have dermal bony plates or scutes, which are even in some cases prolonged into formidable spines, and others have spur-like processes on the fore-feet. The limb-bones are in some cases solid, in others hollow; and the teeth are serrated, pointed and recurved in the carnivorous, and worn flat in the herbivorous kinds. Some forms seem to have been furnished with horny beaks. Some of the three-toed footprints in the Triassic rocks of America belong to Dinosaurs; but the group apparently culminated in the Jurassic. From rocks of this age in Colorado, Marsh obtained Atlantosaurus, the thigh-bone of which is over 6 feet long, so that the whole animal is estimated to have been 80 feet long and 30 feet high, the largest land-animal known in evny age. Marsh considers that the dinosaurs should rank as a subclass, and divides them into orders, of which the chief are the Sauropoela, Stegosauria, Omithopoda, and Theropoda. The Sauropoda are herbivorous with nearly equal limbs, solid limb-bones, and five claws on each of the plantigrade feet. This order includes the least specialised and most crocodilian forms, such as Atlantosanrns and Brontosaurns, as well as the huge British forms known as Cetiosaurus. The Stegosauria differ in having very small fore limbs and osseous dermal armour, often spinous, as in Scelidosaurus from the Lias of Lyme Regis, Polacanthus and Hgleeosanrus from the Wealden, and Acanthopholis from the Chalk. The Omithopoda have small fore limbs with five digits each, hollow hind limb-bones, with only three or four toes to each digitigrade foot, and no teeth in the front of the jaw. This order includes the small arboreal or rock-climbing Hypsilophodon and the probably amphibious Iguanodon (q.v.). The Theropoda have small fore limbs, hollow limb-bones, digitigrade feet with prehensile claws and teeth indicating their carnivorous character. Megalosaurus and the Triassic Thecodontosaurus are among the best known European genera.