This order includes a number of mainly terrestrial, extinct reptiles, which differ much from one another, and show remarkable points of affinity on the one hand with the Labyrinthodont Amphibia, and on the other with the Mammalia. The vertebrae are nearly always amphicoelous and sometimes have notochordal centra. The skull is short and has the quadrate immovably fixed. There is an interparietal foramen, and generally large supratemporal fossae bounded by supratemporal[2] arcades, but with no infratemporal[69] arcades; _Elginia_ however has the whole of the temporal region completely roofed over. The teeth are placed in distinct sockets and are very variable in form, the dentition sometimes resembling the heterodont dentition of mammals. The humerus has distinct condyles and an ent-epicondylar foramen[70] as in many mammals. The pubis is fused with the ischium, and both pectoral and pelvic girdles are remarkably solid. The obturator foramen is remarkably small or even absent. The anterior ribs have two articulating surfaces, and each articulates by its tuberculum with the transverse process, and by its capitulum with the centrum as in mammals. These reptiles occur chiefly in deposits of Triassic and Permian age. Some of the best known genera are _Dicynodon_, _Udenodon_, _Placodus_, _Pariasaurus_ and _Galesaurus_. They will be noticed in the general account of the skeleton in reptiles. _Order 2._ SAUROPTERYGIA. This order includes a number of extinct marine reptiles, devoid of an exoskeleton. The tail is short, the trunk long, and the neck in the most typical forms extremely long. The vertebrae have slightly biconcave, or nearly flat centra. The skull is relatively small and has large supratemporal fossae. The teeth are placed in distinct sockets, and are generally confined to the margins of the jaws; they are sharp and curved and are coated with grooved enamel. The premaxillae are large, and there is an interparietal foramen. The quadrate is firmly united to the cranium. The anterior nares are separate and are placed somewhat close to the orbits. There is no ossified sclerotic ring. The palatines and pterygoids meet the vomers, and more or less completely close the palate, and in some forms, e.g. _Plesiosaurus_, there is a distinct parasphenoid. Thoracic ribs are strongly developed and each articulates with its vertebra by a single head. The cervical vertebrae have well-marked ribs, which articulate only with the centra, in this respect differing from those of Crocodiles. The caudal vertebrae bear both ribs and chevron bones, and abdominal splint-ribs are largely developed.