The vegetarian Dinosaurs first became known in this system, but their development was so extraordinary that they soon outranked the flesh-eaters both in size and diversity. Among these the _Brontosaurus_ attained the extraordinary length of sixty feet, and possibly more. It walked on its four legs, and is one of the largest known of all land animals. This enormous creature in spite of all its size and bulk was yet rather weak than strong. Its general organisation was unwieldy; the head was very small, and the brain hardly bigger than a walnut. The task of providing food for such a body must have been a severe tax on so small a head. The inconvenience of its bulkiness was perhaps reduced by living in and about water; but from the excellent preservation of some of the skeletons it has been thought that its life was often ended by sinking in some quicksand or shoal, from which its own massiveness forbade that the _Brontosaurus_ should extricate itself. [Illustration: Ornitholestes Diplodoci Carnegiei From skeletons found in Jurassic strata in Wyoming, U.S.A. (These reptiles attained a length of about 80 feet.)] Not greatly removed in habit or appearance from the _Brontosaurus_ was the _Diplodocus_, a magnificent specimen of which has been set up in Pittsburg, and a fine replica, owing to the generosity of Mr. Andrew Carnegie, in the Natural History Museum. The _Diplodocus_, a harmless placid beast, was over eighty feet from the tip of his snout to the end of his enormous tail. It has been calculated that impulses travel along the nerves to the brain at the rate of about twelve yards a second. The rate may have been less in the case of the sluggish _Diplodocus_, but in any case it would evidently take at least two seconds for a nerve impulse to travel the length of this reptile; so that if any enemy attacked him at the end of his tail it would be two seconds before the _Diplodocus_ would realise the fact, and perhaps four seconds before he could begin to turn round to defend himself. Even larger than these was the tremendous _Brachiosaurus_, who weighed as much as a steam-engine and whose thigh-bone was nearly eight feet high. These were the largest reptiles ever known, and may be taken as reaching the point when bulk becomes a burden, and as signalising an approach to the limit of evolution in the line of size. Less bulky than these were the _Stegosaurs_, which were also four-footed. They were curiously armoured, and formed a group of very remarkable creatures found in England and Western America. While they were less gigantic than some of those we have just described, they found compensation in protective plates, spines, and similar modes of defence. A _Stegosaurus_ found in Wyoming was probably the most hideous to look upon; but like his relatives he had an extraordinarily small head and brain, and was a sluggish creature depending on his ugliness and armour for protection. Very likely this small size of the brain of