[Illustration: Fig. 25--Skeleton of _Stegosaurus ungulatus_; length about 25 feet. (After Marsh.)] [Illustration: Plate X. A GIGANTIC ARMOURED DINOSAUR, STEGOSAURUS UNGULATUS. Length about 30 feet.] [Illustration: Fig. 26.--Tail vertebræ of _Stegosaurus_. (After Marsh.) 1. Side view. 2. Front view.] Finally, the Stegosaur displays a rather remarkable feature; for a very large chamber was found in the sacrum[26] formed by an enlargement of the spinal cord. The chamber strongly resembled the brain-case in the skull, but was about ten times as large! So this anomalous monster had two sets of brains, one in its skull, and the other in the region of its haunches! and the latter, in directing the movements of the huge hind limbs and tail, did a large share of the work. The subject is a highly suggestive one, but at present requires further explanation. [26] The sacrum may be thus defined: the Vertebræ (usually fused together) which unite with the haunch-bones (_ilia_) to form the pelvis. [Illustration: Fig. 27.--Limb-bones of _Stegosaurus_. (After Marsh.) 1. Fore leg. 2. Hind leg.] On the walls of the fossil reptile gallery at South Kensington the reader will find a large framed drawing of the skeleton of Stegosaurus, kindly sent by Professor Marsh, whose forthcoming monograph will be welcomed by all palæontologists. [Illustration: Fig. 28.--1, 2. Plates of Stegosaurus. The middle figures show their thickness. (After Marsh.)] [Illustration: Fig. 29.-Head of _Triceratops_, seen from above. (After Marsh.)] The last, and in some ways the strangest of the Dinosaurs, was the Triceratops[27] that flourished in America at the end of the long Mesozoic era, during the Cretaceous period. The name refers to the three horn-cores found on the skull, which probably supported true horns like those of oxen. Whereas the Stegosaur was provided with quite a small skull, this monster had one of huge dimensions and remarkable shape (see Figs. 29 and 30).[28] In the younger ones it was about six feet long, but in an old individual must have reached a length of seven or eight feet. Such a skull is only surpassed by some whales of the present day. Twenty different skulls of this kind have been found, and Professor Marsh places the horned Dinosaurs in a separate family, to which he has given the name Ceratopsidæ, or horn-faced. Their remains come from the Laramie beds, believed to be of Cretaceous age, but representing a remarkably mixed fauna and flora, so that some have considered them to be Tertiary. The strata containing these fossils are very rich in organic remains, and have yielded not only other Dinosaurs, but Plesiosaurs, crocodiles, turtles, many small reptiles, a few birds, fishes, and small mammals. The Ceratops beds are of fresh-water or brackish origin, and can now be traced for nearly eight hundred miles along the east flank of the Rocky Mountains. [27] Greek--_treis_, three; _ceras_, horn; _ops_, face.