Bones have relations to each other and places in the head which can only change as the organs with which they are associated change their positions. No matter what the position of a nostril may be--at the extremity of a long snout, as in an ant-eater, or far back at the top of the head in a porpoise, or at the side of the head in a bird--it is always bordered by substantially the same bones, which vary in length and size with the changing place of the nostril and the form of the head. Every region of the head is defined by this method of construction; so that eye holes and nose holes, brain case and jaw bones, palate and teeth, beak, and back of the skull are all instructive to those who seek out the life-history of these animals. We may briefly examine the head of an Ornithosaurian. BONES ABOUT THE NOSTRIL No matter what its form may be, the head of an Ornithosaur always terminates in front in a single bone called the intermaxillary. It sends a bar of bone backward above the visible nostrils, between them; and a bar on each side forms the margin of the jaw in which teeth are implanted. The bone varies in depth, length, sharpness, bluntness, slenderness, and massiveness. As the bone becomes long the jaw is compressed from side to side, and the openings of the nostrils are removed backward to an increasing distance from the extremity of the beak. The outer and hinder border of the nostril is made by another bone named the maxillary bone, which is usually much shorter than the premaxillary. It contains the hindermost teeth, which rarely differ from those in front, except in sometimes being smaller. The nasal bones, which always make the upper and hinder border of the nostrils, meet each other above them, in the middle line of the beak. [Illustration: FIG. 20 Showing that the extremity of the jaws in Rhamphorhynchus was sheathed in horn as in the giant Kingfisher, since the jaws similarly gape in front. The hyoid bones are below the lower jaw in the Pterodactyle.] The nostrils are unusually large in the Lias genus named Dimorphodon, and small in species of the genus Rhamphorhynchus from Solenhofen. Such differences result from the relative dimensions and proportions of these three bones which margin the nasal vacuity, and by varying growth of their front margins or of their hinder margins govern the form of the snout. The jaws are most massive in the genera known from the Wealden beds to the Chalk. The palatal surface is commonly flat or convex, and often marked by an elevated median ridge which corresponds to a groove in the lower jaw, though the median ridge sometimes divides the palate into two parallel concave channels. The jaw is margined with teeth which are rarely fewer than ten or more than twenty on each side. They are sharp, compressed from side to side, curved inward, and never have a saw-like edge on the back and front margins. No teeth occur upon the bones of the palate.