But the identity of the pneumatic foramina in Birds and Flying Reptiles is not a character which stands by itself as evidence of organisation, for a mould of the form of the brain case contributes evidence of another structural condition which throws some light on the nature of Ornithosaurs. Among many of the lower animals, such as turtles, the brain does not fill the chamber in the dry skull, in which the same bones are found as are moulded upon the brain in higher animals. For the brain case in such reptiles is commonly an envelope of cartilage, as among certain fishes; and except among serpents, the Ophidia, the bones do not completely close the reptilian brain case in front. The brain fills the brain case completely among birds. A mould from its interior is almost as definite in displaying the several parts of which it is formed as the actual brain would be. And the chief regions of the brain in a bird--cerebrum, optic lobes, cerebellum--show singularly little variation in proportion or position. The essential fact in a bird's brain, which separates it absolutely from all other animals, is that the pair of nerve masses known as the optic lobes are thrust out at the sides, so that the large cerebral hemispheres extend partly over them as they extend between them to abut against the cerebellum. This remarkable condition has no parallel among other vertebrate animals. In Fishes, Amphibians, Reptiles, and Mammals the linear succession of the several parts of the brain is never departed from; and any appearance of variation from it among mammals is more apparent than real, for the linear succession may be seen in the young calf till the cerebral hemispheres grow upward and lop backward, so as to hide the relatively small brain masses which correspond to the optic lobes of reptiles, extending over these corpora-quadrigemina, as they are named, so as to cover more or less of the mass of the cerebellum. From these conditions of the brain and skull, it would not be possible to mistake a mould from the brain case of a bird for that of a reptile, though in some conditions of preservation it is conceivable that the mould of the brain of a bird might be distinguished with difficulty from that of the brain in the lowest mammals. Taken by itself, the avian form of brain in an animal would be as good evidence that its grade of organisation was that of a bird as could be offered. THE BRAIN IN SOLENHOFEN PTERODACTYLES It happens that moulds of the brain of Pterodactyles, more or less complete, are met with of all geological ages--Liassic, Oolitic, and Cretaceous. The Solenhofen Slate is the only deposit in Europe in which Pterodactyle skulls can be said to be fairly numerous. They commonly have the bones so thin as to show the form of the upper surface of the mould of the brain, or the bones have scaled off the mould, or remain in the counterpart slab of stone, so as to lay bare the shape of the brain mass.