Order 4. Ornithischia (Deinosauria pars). Archosauria of herbi vorous diet. The preorbital fossa is usually small or absent, the nostrils very large. The quadrate, unless secondarily fixed, is movable, a spherical head on its upper extremity resting in a cup in the squamosal. Premaxillae usually toothless, and covered with a horny beak, which opposes a similar structure carried by a special predentary bone in the lower jaw. Posterior end of the dentary raised into an upstanding coronoid process. The pubis bifid, a prepubic process stretching forward along the belly, and a posterior part passing downward and backward parallel to the ischium. Ilium elongated anteroposteriorly. Acetabulam per forate. Fore leg shorter than the hind, the animal being often bipedal. Hand usually pentadactyl, foot often tridactyl.
Rhaetic to Upper Cretaceous.
Super-family Ornithopodidae. Families : Hypsilophodontidae, Camptosauridae, Ignanodontidae, Trachodontidae. Super-family Stegosauridae. Families : Scelidosauridae, Stegosauridae, Acantha pholidae, Polocanthidae. Super-family Ceratopsidae, with one family.
Order 5, Pterosauria. Archosauria fully adapted for flight. The vertebrae and many long bones are hollow and, where occupied by air sacs, arising no doubt, like those of birds, by extension of the bronchi. Skull elongated, triangular in plan, and peculiar in that the quadrato-jugal excludes the jugal from the border of the infra, temporal fossa. Teeth may extend throughout the jaws, be re stricted to their anterior ends, or be absent altogether.
Cervical vertebrae large, procoelous, and very freely movable, head carried nearly at right angles to the neck. Dorsal vertebrae small, sometimes largely fused, sacrum of four to HD vertebrae, tail either very short or greatly elongated and quite stiff.
Scapula and coracoids elongated slender rods, the latter articu lating with a large shield-shaped sternum. Clavicular arch absent. Ilium long, pubis and ischium fused with it and with each other, not meeting in a median symphysis. Prepubic bones present. Fore limb supporting a wing, which is formed by a fold of skin arising from the side of the body and stretched between the upper arm, fore arm and greatly extended fourth finger, and the hind leg. Fingers one to three, present and clawed.
Sub-order Rhamphorhynchoidae. Pterosaurs, with a long tail, wing metacarpal short, fifth toe well developed.
Upper Trias? Lower Lias to Upper Jurassic.
Sub-order Pterodactyloidae. Pterosaurs, with a short tail, wing metacarpal long, and fifth toe reduced or absent.
Upper Jurassic to Upper Cretaceous. Families : Pterodactylidae, Ornithocheiridae.
The birds, class Ayes, are certainly descendants of Archosaurian reptiles; had the group become extinct in Cretaceous time it would be regarded as an order equivalent to those listed above.
The remaining reptilian orders cannot usefully be grouped into super-orders.
Order Rhynchocephalia. Reptiles in which the temporal region is perforated by two fossae; the supratemporal fossa seems to differ from that of Archosauria in that the post frontal enters into its margin, whilst the infratemporal fossa differs by the exclusion of the quadratojugal.
The preorbital part of the skull is short, and there is no pre orbital opening. The fenestra ovalis is placed high in the skull. The dentary bears a single series of acrodont teeth which bite into a groove between the similar teeth on the maxilla and palatine, so that with use they acquire a wedge-shaped section. The verte brae have amphicoelous centra, and all the ribs are single-headed. An unossified sternum is present. The shoulder girdle includes scapulae, precoracoids, clavicles and an interclavicle.
The pelvis has an ilium attached to two sacral vertebrae and directed downward in front. The pubis and ischia are plate-like in primitive forms, but diverge widely in later times. The limbs are pentadactyl, and the fifth metatarsal has a hook-shaped upper extremity.
One group of Rhynchocephalia, the Champsosauridae, became highly adapted to an aquatic life in estuaries.
Middle Trias to Recent. Families : Rhynchosauridae, Saurano: dontidae, Sphenodontidae, Champsosauridae.
Order Squamata. (The following account does not include the characters of the reptile Pleurosaurus, which is, perhaps, a mem ber of the order.) Reptiles in which the dermal roof of the temporal region is so far reduced that only a single temporal arcade, or none at all, remains. The quadrate is thereby freed so that it can move, its rounded head articulating with one or two bones which are con nected with the parietal. If two bones be present the inner is firmly applied to the front face of the posterior wing of the parietal, and rests against and may even be firmly fixed by suture to the front face of the end of the paroccipital process. This bone is either the supratemporal or squamosal, or, very improbably, tabular. The outer bone is fixed to the lateral surface of the inner, often overlapping it on to the parietal ; it stretches forward as the hinder part of the temporal arcade, and meets the postorbital and sometimes the jugal. This bone is either the squamosal or quad rato-jugal. In the palate the pterygoid no longer reaches the prevomers, and the whole is often very lightly constructed.