Stegosaurus (plated reptile) takes its name from the double row of bony plates arranged along its back.  The powerful tail was armed with long spines, and the thick skin was defended with irregular bits of bone implanted in it.  The brain of the stegosaur was smaller than that of any land vertebrate, while in the sacrum the nerve canal was enlarged to ten times the capacity of the brain cavity of the skull.  Despite their feeble wits, this well-armored family lived on through millions of years which intervened between their appearance, at the opening of the Jurassic, and the close of the Cretaceous, when they became extinct.

A less stupid brute than the stegosaur was triceratops, the dinosaur of the three horns,—­one horn carried on the nose, and a massive pair set over the eyes.  Note the enormous wedge-shaped skull, with its sharp beak, and the hood behind resembling a fireman’s helmet.  Triceratops was fully twenty-five feet long, and of twice the bulk of an elephant.  The family appeared in the Upper Cretaceous and became extinct at its close.  Their bones are found buried in the fresh-water deposits of the time from Colorado to Montana and eastward to the Dakotas.

Marine reptiles.  In the ocean, reptiles occupied the place now held by the aquatic mammals, such as whales and dolphins, and their form and structure were similarly modified to suit their environment.  In the Ichthyosaurus (fish reptile), for example, the body was fishlike in form, with short neck and large, pointed head (Fig. 333).

A powerful tail, whose flukes were set vertical, and the lower one of which was vertebrated, served as propeller, while a large dorsal fin was developed as a cutwater.  The primitive biconcave vertebrae of the fish and of the early land vertebrates were retained, and the limbs degenerated into short paddles.  The skin of the ichthyosaur was smooth like that of a whale, and its food was largely fish and cephalopods, as the fossil contents of its stomach prove.

These sea monsters disported along the Pacific shore over northern California in Triassic times, and the bones of immense members of the family occur in the Jurassic strata of Wyoming.  Like whales and seals, the ichthyosaurs were descended from land vertebrates which had become adapted to a marine habitat.

PLESIOSAURS were another order which ranged throughout the Mesozoic.  Descended from small amphibious animals, they later included great marine reptiles, characterized in the typical genus by long neck, snakelike head, and immense paddles.  They swam in the Cretaceous interior sea of western North America.

MOSASAURS belong to the same order as do snakes and lizards, and are an offshoot of the same ancestral line of land reptiles.  These snakelike creatures—­which measured as much as forty-five feet in length—­abounded in the Cretaceous seas.  They had large conical teeth, and their limbs had become stout paddles.