Clavicles and an interclavicle are always present, the pelvis is plate-like, and there are only two sacral vertebrae.
Upper Permian to Upper Trias. Families : Eosuchidae, Phy tosauridae, Pseudosuchidae, Erythrosuchidae, Erpetosuchicke and others not yet defined.
This order is, in a sense, artificial, it includes the ancestors, for the greater part unknown, of the remainder of the orders of Archosauria, and in addition contains a number of animals which belong to short-lived unsuccessful side branches.
Order 2. Crocodilia. Archosaurs usually of medium or large size, and adapted more or less completely to an aquatic habit. The skull is characterized most clearly by the fact that the quadrate is very large, and lies at a very low angle with the horizontal. The wedge-shaped otic cavity so formed is closed behind by a downgrowth of the squamosal, which, with the overlapping "ex occipital" reaches the quadrate. The tympanic membrane lies some distance below the outer surface, and the external auditory meatus can be closed by a muscular flap. The elongated face is chiefly formed by the maxillae, the external nostrils, usually con fluent in the bony skull lying quite anteriorly. There is always a secondary palate, the choanae lying posteriorly between the pala tics or pterygoids. The vertebrae are amphiplatean or procoelous, the ribs double-headed throughout the presacral part of the col umn, the dorsal ribs articulating entirely with the neural arch. The coracoid is elongated, clavicles are absent ; and the sternum is unossified. The ilium is a small bone supported by two sacral ribs, and the pubis is excluded from the acetabulum.
The hand is five-fingered, the foot has the fifth toe reduced to a stump of its metatarsal.
Lower Jurassic (Upper Trias) to Recent. Families : Teleosauri dae, Metriorhynchidae, Dyrosauridae, Goniopholidae, Libycosu chidae, Pholidosauridae, Stomatosuchidae, Gavialidae, Croco dilidae.
It is not improbable that the Crocodilia sprang from the family Erpetosuchidae of the order Thecodontia.
Order 3. Saurischia (Deinosauria parts) Archosauria, with a well-developed preorbital vacuity. The neck is sharply marked off
from the trunk. The presacral ribs are two-headed, and the dorsal ribs articulate only with the neural arch. Clavicle and inter clavicle are lacking, the coracoid is short. There are three or more sacral vertebrae. The pubis and ischia form diverging rods, primi tively the pelvis is plate-like, but the bones separate from one another in later forms. The acetabulum is perforate. The fore limb is shorter than the hind, and the femur moves in a plane parallel to the animal's length. The body is thus held well above the ground, and the animal is often bipedal.
Sub-order Theropoda. Carnivorous Saurischia, in which the dentition consists of a single series of the codont, laterally com pressed teeth in the premaxillae and maxillae. The cervical verte brae may be opisthocoelus. The fore limb is often very much smaller than the hind, and the animals are usually bipedal. The hand tends to be reduced to the first three fingers, which are provided with powerful claws, and the foot becomes functionally tridactyl and symmetrical about the third toe.
Middle Trias to Upper Cretaceous.
Families : Hallopidae, Podokosauridae, Coeluridae, Compsog natkidae, Ornithomimidae, Plateosauridae, Zanclodontidae, An chisauridae, Megalosauridae, Spinosauridae.
Sub-order Sauropoda. Herbivorous Saurischia, usually of gigan tic size. The skull is extremely small, and the dentition feeble. The cervical and many or all the dorsal vertebrae opisthocoelic, Accessory articulating faces are developed in the neural arch of the dorsal vertebrae. The dorsal centra are excavated laterally, so that they may be reduced to mere shells of bones.
The animals are quadripedal, and walk on the ends of the meta podials, both feet are five toed, but some of the digits have a reduced number of phalanges and most lack claws.
Middle Jurassic to Upper Cretaceous. Families : Cetrosauridae, Allantosauridae, Camarosauridae, Diplodocidae, Titanosuchidae.