In most birds there is a large vacuity in the side of the head between the nostril and the orbit of the eye, partly separated from it by the bone which carries the duct for tears named the lachrymal bone. The same preorbital vacuity is present in all long-tailed Pterodactyles, though it is either less completely defined or absent in the group with short tails. It affords excellent distinctive characters for defining the genera. In the long-tailed genus Scaphognathus from Solenhofen this preorbital opening is much larger than the nostril, while in Dimorphodon these vacuities are of about equal size. Rhamphorhynchus is distinguished by the small size of the antorbital vacuity, which is placed lower than the nostril on the side of the face. The aperture is always imperfectly defined in Pterodactylus, and is a relatively small vacuity compared with the long nostril. In Ptenodracon the antorbital vacuity appears to have no existence separate from the nostril which adjoins the eye hole. And so far as is known at present there is no lateral opening in advance of the eye in the skull in any Ornithosaur from Cretaceous rocks, though the toothless Ornithostoma is the only genus with the skull complete. When a separate antorbital vacuity exists, it is bordered by the maxillary bone in front, and by the malar bone behind. The prefrontal bone is at its upper angle. That bone is known in a separate state in reptiles and, I think, in monotreme mammals. Its identity is soon lost in the mammal, and its function in the skull is different from the corresponding bone in Pterodactyles. BONES ABOUT THE EYES [Illustration: FIG. 21. UPPER SURFACE OF SKULL OF THE HERON Compared with the same aspect of the skull of Rhamphorhynchus] The third opening in the side of the head, counting from before backward, is the orbit of the eye. In this vacuity is often seen the sclerotic circle of overlapping bones formed in the external membrane of the eye, like those in nocturnal birds and some reptiles. The eye hole varies in form from an inverted pear-shape to an oblique or transverse oval, or a nearly circular outline. It is margined by the frontal bone above; the tear bone or lachrymal, and the malar or cheek bone in front; while the bones behind appear to be the quadrato-jugal and post-frontal bones, though the bones about the eye are somewhat differently arranged in different genera.