The most impressive facts in the Pterodactyle skeleton, in view of these affinities, are the structures which it has in common with Reptiles. Some structures are fundamental, like the cup-and-ball articulation of the vertebræ, which is never found in birds or mammals. Although not quite identical with the condition in any Reptile, this structure is approximately Lizard-like or Crocodile-like in the cup-and-ball character. It shows that the deepest-seated part of the skeleton is Reptile-like, though it may not be more Reptilian than is the vertebral column of a Mammal, if comparison is made between Mammals and extinct groups of animals known as Reptiles, such as Dinosaurs and Theriodontia. The orders of animals which have been included under the name Reptilia comprise such different structural conditions of the parts of the skeleton which may be termed reptilian in Ornithosaurs, that there is good reason for regarding the cup-and-ball articulation as quite a distinctive Reptilian specialisation, in the same sense that the saddle-shaped articulation between the bodies of adjacent vertebræ in a bird is an Avian specialisation. From the theoretical point of view the Ornithosaur acquired its Reptilian characters simultaneously with its Avian and Mammalian characters. There is nothing in the structure of the skeleton of the Dinosauria, to which Ornithosaurs approximate in several parts of the body, which would help to explain the cup-and-ball articulation of the backbone, if the Flying Reptile were supposed to be an offshoot from the carnivorous Dinosaurs. The elimination of Reptile characters from so much of the skeleton, and the substitution for them of the characters of Birds and Mammals, would be of exceptional interest if there had been any ground for regarding the flying animal as more nearly related to a Reptile than to a Bird. But if the evidence from the form of the brain and nature of the pneumatic organs seen in the limb bones accounts for the Avian features of the skeleton, the Reptilian condition of the vertebral column helps to show a capacity for variation, and that the fixity of type and structure, which the skeleton of the modern Bird has attained, is not necessarily limited to or associated with the vital organs of Birds. The variation of the cup-and-ball articulation in the neck of a Chelonian, which makes the third vertebra cupped behind, the fourth bi-convex, the fifth cupped in front, and the sixth flattened behind, shows that too much importance may be attached to the mode of union of these bones in Serpents, Crocodiles, and those Lizards which have the cup in front; for while in Lizards the anterior cup, oblique and depressed, is found in most of its groups, the Geckos show no trace of the cup-and-ball structure, and in that respect resemble the Hatteria of New Zealand.