In this reduction or fusion of parts, or in addition thereto, there has been a general lightening-up of the whole skull-structure in reptiles from the rather massive and protected form of the older to the lighter, less protected, and more fragile type of the later ones, since speed, greater agility, better sense organs, and doubtless greater brain power have rendered unnecessary or useless the older kinds, just as modern methods and modern arms have rendered useless the coat of mail of the Middle Ages. [Illustration: FIG. 7.—_Edaphosaurus_, a theromorph reptile from the Permian of Texas. Skull with single temporal vacuity.] The old reptiles had a continuous covering or roof for the skull, pierced only by the openings for the nostrils in front—the nares—the orbits for the eyes near the middle, and a smaller median opening back of them for the so-called “pineal eye.” The temporal region, that is, the region back of the orbits on each side, was completely roofed over by bone for the support and protection of the jaw muscles. In later reptiles this region has been lightened, either by holes that pierce it or by the emargination of its free borders, as in the turtles. The openings have occurred in different ways, and with the loss of different bones in various lines of descent. In one large group of reptiles, comprising the pterodactyls, dinosaurs, phytosaurs, crocodiles, and rhynchocephalians, there are two openings on each side, called the supratemporal and lateral temporal vacuities. In another still larger group there is a single vacuity on each side, all members of which it has been thought were markedly related to each other. Some of these, the lizards, snakes, and mosasaurs, the ichthyosaurs, and probably the proganosaurs, have the single opening high up on the side, corresponding apparently to the supra temporal vacuity of the double-arched forms, as those with two openings are called. Many others, however, like the whole order Therapsida and the Theromorpha, have the single opening lower down and bounded differently; their relationships are doubtful, since it is very much of a question how the single opening has arisen. There have been many theories to account for the origin of the temporal vacuities, but all are yet speculations. Notwithstanding these doubts, which more recent discoveries have intensified, there can be none that the structure of this region of the skull offers important and reliable characters for the classification of the reptiles into the larger groups, but, unfortunately, we are very uncertain yet as to what this classification should be. We are confident that all those reptiles having two temporal vacuities on each side are related to each other; we are yet very much in doubt as to the classification of all other reptiles, or at least all others having only a single temporal vacuity on each side.