In the head, Cambridge specimens show no trace of the parts which are between the brain-cavity and the fore-part of the jaw. The form and condition of the orbits, nares, and of the space between them, vary in German specimens. Some Birds and certain Ruminants, such as deer, the giraffe, &c., have an interspace between the orbits and nares corresponding to that in some Pterodactyles, but no such perforation is found in living reptiles. In mammals it appears to be surrounded by the frontal, nasal, lachrymal, and often by the maxillary bone. In birds the bones appear to be the lachrymal, nasal, maxillary and premaxillary, as is the case with Pterodactyles, except that the nasal bones would seem sometimes to be excluded. The chief peculiarity of the Pterodactyle skull in this region is made by the malar bone (and, according to some authors, the maxillary also) sending up a process to meet the lachrymal. This is not seen in birds, but is characteristic of many mammals and reptiles. The premaxillary bone is single, as in birds and Iguana; but it appears to attain as great a development as in birds, and to occupy the portion of the jaw which among reptiles and mammals is made by the maxillary bone. Owing to the great development of the premaxillary bones, the exterior nares are placed far back toward the middle of the skull as in birds, and not near the tip of the snout as in living reptiles and most mammals. The orbits in Pterodactyles are surrounded with bone, as is commonly the case with mammals and reptiles. Among birds a complete orbit is seen among the parrots, in which it is completed below by a prolongation of the outer posterior corner of the frontal, which would correspond to the post-frontal bone, and by the lachrymal bone. Thus the malar bone, which in most mammals and reptiles forms an important part of the lower margin of the orbit, is in birds entirely excluded. In Pterodactyles the malar bone is placed between the lachrymal and the post-frontal process of the frontal bone.