they could probably perch like birds on rocks and trees. Their heads, though very lightly framed, were large and reptilian in aspect, and furnished with sharp teeth, and sometimes probably with a beak as well. Few creatures of the old world are of more hideous and sinister aspect. Yet some of them must have been as light and graceful on the wing as swallows or sea-gulls. There are many species, most of them small, but some of those in the later Mesozoic attained to so great a size that the expanse of their wings must have exceeded twenty feet, making them veritable flying dragons, probably formidable to all the smaller animals of their time. Though these animals were strictly reptiles, they combined in their structures contrivances for aërial locomotion now distributed between the bats and the birds. They had bat-like wings and bird-like chests. Some had horny beaks. All had hollow limb bones, and air cavities to give lightness to the skull. Their brains approach to those of birds, and, as already stated, their respiration and circulation must have been of a high order. These facts are very suggestive, and perhaps in no point is the imagination or the faith of the devout evolutionist more severely tested than in realising the spontaneous assumption of these characters by reptiles, and their subsequent distribution between the very dissimilar types in which they are now continued. [Illustration: FIG. 148.—A Jurassic Bird (_Archæopteryx macroura_).—After Owen.] [Illustration: FIG. 149.—Jaw of a Cretaceous Toothed Bird (_Ichthyornis dispar_).—After Marsh. Natural size.]