The eyes were frequently, if not always, in contact with the anterior walls of the brain case, as in many birds, and are always far back in the side of the head. In Dimorphodon they are in front of the articulation of the lower jaw; in Rhamphorhynchus, above that articulation; while in Ornithostoma they are behind the articulation for the jaw. This change is governed by the position of the quadrate bone, which is vertical in the Lias genus, inclined obliquely forward in the fossils from the Oolites, and so much inclined in the Chalk fossil that the small orbit is thrown relatively further back. Thus far the chief difference in the Pterodactyle skull from that of a bird is in the way in which the malar arch is prolonged backward on each side. It is a slender bar of bone in birds, without contributing ascending processes to border vacuities in the side of the face, while in these fossil animals the lateral openings are partly separated by the ascending processes of these bones. This divergence from birds, in the malar bone entering the orbit of the eye is approximated to among reptiles and mammals, though the conditions, and perhaps the presence of a bone like the post-orbital bone, are paralleled only among Reptiles. The Pterodactyles differ among themselves enough for the head to make a near approach to Reptiles in Dimorphodon, and to Birds in Pterodactylus. In the Ground Hornbill and the Shoebill the lachrymal bones in front of the orbits of the eyes grow down to meet the malar bars without uniting with them. The post-frontal region also is prolonged downward almost as far as the malar bar, as though to show that a bird might have its orbital circle formed in the same way and by the same bones as in Pterodactylus. Cretaceous Ornithosaurs sometimes differ from birds apparently in admitting the quadrato-jugal bone into the orbit. It then becomes an expanded plate, instead of a slender bar as in all birds. THE TEMPORAL FOSSA A fourth vacuity is known as the temporal fossa. When the skull of such a mammal as a Rabbit, or Sheep, is seen from above, there is a vacuity behind the orbits for the eyes, which in life is occupied by the muscles which work the lower jaw. It is made by the malar bone extending from the back of the orbit and the process of bone, called the zygomatic process, extending forward from the articulation of the jaw, which arches out to meet the malar bone. In birds there is no conspicuous temporal fossa, because the malar bar is a slender rod of bone in a line with the lower end of the quadrate bone.