The Ornithosauria are one of a series of groups of animals, living and extinct, which have been combined in an alliance named the Ornithomorpha. That group includes at least five great divisions of animals, which circle about birds, known as Ornithosauria, Crocodilia, Saurischia, Aves, Ornithischia, and Aristosuchia. Their relations to each other are not evident in an enumeration, but may be shown in some degree in a diagram (see p. 190). THE ORNITHOMORPHA The Ornithomorpha arranged in this way show that the three middle groups--carnivorous Saurischia, Aristosuchia, herbivorous Ornithischia--which are usually united as Dinosauria, intervene between Birds and Ornithosaurs; and that the Crocodilia and Ornithosauria are parallel groups which are connected with Birds, by the group of Dinosaurs, which resembles Birds most closely. The Ornithomorpha is only one of a series of large natural groups of animals into which living and extinct terrestrial vertebrata may be arranged. And the succeeding diagram may contribute to make evident the relations of Ornithosauria to the other terrestrial vertebrata (see p. 191). Herein it is seen that while the Ornithomorpha approach towards Mammalia through the Ornithosauria, and less distinctly through the Crocodilia, they approach more directly to the Sauromorpha, through the Plesiosaurs and Hatteria; while that group also approaches more directly to the Mammals through the Plesiosaurs and Anomodonts. [Illustration: DIAGRAM OF THE AFFINITIES OF THE ORDERS OF ANIMALS COMPRISED IN THE ORNITHOMORPHA. After a diagram in the _Philosophical Transactions of the Royal Society_, 1892.] The Aristosuchia is imperfectly known, and therefore to some extent a provisional group. It is a small group of animals. [Illustration: DIAGRAM SHOWING THE RELATIONS OF THE ORNITHOMORPHA TO THE CHIEF LARGE GROUPS OF TERRESTRIAL VERTEBRATA, AND THEIR AFFINITIES WITH EACH OTHER. After a diagram in the _Philosophical Transactions of the Royal Society_, 1892.] Cordylomorpha are Ichthyosaurs and the Labyrinthodont group. Herpetomorpha include Lacertilia, Homoeosauria, Dolichosauria, Chameleonoidea, Ophidia, Pythonomorpha. The Sauromorpha comprises the groups of extinct and living Reptiles named Chelonia, Rhynchocephala, Sauropterygia, Anomodontia, Nothosauria, and Protorosauria. These details may help to explain the place which has been given to the Ornithosauria in the classification of animals. [Illustration: FIG. 75. COMPARISON OF SIX GENERA The skulls are seen on the left side in the order of the names below them]