STEGOCEPHALIA, one of the primary divisions of the amphibia, sometimes considered as a sub-class, sometimes as an order, especially characterized by the great development of der mal or superficial bones on the dorsal surface of the head. Being totally extinct and conse quently known only from their skeletal remains many questions concerning their relationships continue open. Many of the stegocephali were of salamander-like form, with broad, flattened head, simple, weak paired limbs of the typical pentadactyl type and a more or less elongated tail. Some were lizard-like with stronger limbs. Some, like OPhiderYeton, lacked limbs and were serpentine in form. Many are known to have passed through branchiate larval stages and some, like the Branchiosauridte, retained the gills throughout life. Many of the large labyrinthodonts had rudimentary tails and must have been somewhat frog-like in aspect. One remarkable form, the Dissorophus multicinctus from the Permian formation of Texas, is de scribed by Professor Cope as having borne an armadillo-like carapace on the back. The roofing bones of the head were numerous and covered the temporal fossa as well as the cranium. In some of the larger forms they were much sculp tured and excavated for the accommodation of mucous canals. The large eyes were supported by a ring of sclerotic bones, found well de veloped at the present day only in the eyes of some birds. In the middle of the cranial roof between the two parietal bones was the con spicuous pineal foramen supposed to have lodged a functional median eye. The nostrils were situated near the tip of the snout and the internal nares in the anterior part of the palate. The base of the skull was imperfectly ossified, but there was a well-developed hasi-occipital bone with usually two occipital condyles. This deficiency of bone was supplied by the large superficial parasphenoid. The lower jaw was complex and the occurrence of an infradentary bone is especially noteworthy. Teeth of a sim ple, conical, partly hollow form, or with the walls fluted or, as in the labyrinthodonta (q.v.), complexly folded, existed on the jaws and palate. The notochord generally persisted and
the vertebra consisted of mere bony shells or were better ossified and formed of alternating intercentra and pleurocentra supporting the hwinal and neural arches or were otherwise segmented. There was never more than a sin gle sacral vertebra. Scapula, coracoid, clavicle and interclavicle were always present in the shoulder girdle and traces of a sternum some times appear. Likewise an ischium, ilium and Occasionally a pubis have been found on each side of the pelvis. Both fore and hind limbs were simple and salamander-like, the carpus and tar sus unossified in the smaller species and pre senting all of the primitive elements with two or three centralia in Archegosaurus and other larger forms. Ribs were simple and never joined the sternum. Some forms like Bran chiosaurus were covered with an armor of scales.
The stegocephali were the first vertebrates to leave the water and assume more or less completely terrestrial habits; they were the first whose limbs departed from the fin type and became pentadactyl; their nasal passages show that they breathed air and they probably had lungs; their teeth indicate a flesh diet, probably consisting chiefly of fish. Footprints of stego cephali are found in abundance in rocks repre senting the muddy shores of the Carboniferous Period and remains of these creatures are found from the Lower Carboniferous through the Permian and in the Triassic in Europe and America. They are especially abundant in the Permian of Texas from which many interest ing forms have been taken. Palmontologists differ on the question of their origin, some con sidering them descendants of the crossopter ygians, others of the Dipnoans. Whatever their origin the stegocephali formed the starting point for all terrestrial and verte brates, probably first giving rise to primitive reptiles like the cotylosauria, becoming them selves extinct. Consult Credner, 'Die Urvier f ; 'Naturw. Woch en sch ri f t (Berlin 1891) ; Cope, 'American Naturalist' (1884) ; Woodward, 'Vertebrate Palaeontology' (Cam bridge 1898); Knipe, H. R., 'Evolution in the Past' (London 1912).