The common hypothetical primary form of all Amniotic animals, which we may call _Protamnion_, and which was possibly nearly related to the Proterosaurus, very probably stood upon the whole midway between salamanders and lizards, in regard to its bodily formation. Its descendants divided at an early period into two different lines, one of which became the common primary form of Reptiles and Birds, the other the primary form of Mammals. Of all the three classes of Amniota, _Reptiles_ (Reptilia, or Pholidota, also called Sauria in the widest sense), remain at the lowest stage of development, and differ least from their ancestors, the Amphibia. Hence they were formerly universally included among them, although their whole organization is much more like that of Birds than Amphibia. There now exist only four orders of Reptiles, namely,—Lizards, Serpents, Crocodiles, and Tortoises. They, however, form but a poor remnant of the exceedingly various and highly developed host of Reptiles which lived during the Mesolithic, or Secondary epoch, and predominated over all other Vertebrata. The immense development of Reptiles during the Secondary epoch is so characteristic that we could as well name it after those animals as after the Gymnosperms (p. 111). Twelve of the twenty-seven sub-orders, given on the accompanying table, and four of the eight orders, belong exclusively to the secondary period. These mesolithic groups are marked by an asterisk. All the orders, with the exception of Serpents, are found fossilized even in the Jura and Trias periods. [Illustration: _Pl. XIV._ [Horizontal axis:] Branches, Classes, and Sub-Classes, of the Vertebrate Stem. Prochordata Evertebrate Forefathers of the Vertebrate Skull-less (Acrania) or Tube-hearted Fish, (Leptocardia) Single nostrilled (Monorrhina) or Round-mouthed Fishes (Cyclostoma) Anamnia { Paired-nostrilled forms or Amphirrhina { with gills, without Amnion. Fish, Pisces Primeval Fish, Selachii. Enamelled Fish, Ganoides. Bony Fish, Teleostei. Mud Fish, Dipneusta. Sea-Dragons, Halisauria. Frogs and Newts, Amphibia. Amniota { Paired nostrilled or Amphirrhina { with Amnion, without gills. Reptiles, Reptilia. Primæval Reptiles, Tocosauria. Lizards, Lacertilia. Snakes, Ophidia. Crocodiles, Crocodilia. Tortoises, Chelonia. Flying Reptiles, Pterosauria. Dragons, Dinosauria. Billed Reptiles, Anomodontia. Birds, Aves. Suckling animals, Mammalia. Billed Animals, Monotrema. Pouched Animals, Marsupialia. Placental Animals, Placentalia. [Vertical axis:] Cenolithic or Tertiary Epoch. Pliocene Age. Miocene Age. Eocene Age.