Zittel, Handbuch der Paleontologie, Bd. III, Abth. 1, p. 397, 1887. Large terrestrial vertebrates; largest of the class. Skull equal to one-fourth or one-third of the entire body in at least one species, _Metoposaurus diagnosticus_ von Meyer (242). Lateral-line canals always present (458) on the skulls as deeply impressed grooves which, in life, were possibly roofed-over by a cartilaginous or other connective-tissue membrane. The sensory organs undoubtedly being supplied by the superficial ophthalmic branch of the trigeminal nerve, branches of which pierced the cranial elements near the grooves, no evidence of such openings in the bottoms of the grooves; the condition probably being analogous to _Hydrolagus colei_ and other chimæroids. Vertebræ stereospondylous, with well-developed neural arches from which projected the well-developed zygapophyses, sometimes slightly amphicœlous and pierced for the notochord, such forms being uncertainly placed in the group. Tail unknown, possibly short. Limbs and girdles well developed (243), phalangeal formula unknown; carpus osseous and tarsus unknown. Pectoral girdle composed of osseous scapulæ, clavicles, interclavicle, coracoid (?); clavicles and interclavicle ventrally sculptured. Pelvic girdle composed of osseous pubis, ischium, and ilium (242), the pubis a small plate, in life largely cartilaginous, the three uniting by cartilaginous union to form the acetabulum. Ventral armature unknown, possibly wanting. Range: Pennsylvanian to Upper Triassic. Distribution: North America, Europe, Asia, Africa, and Australia. =Family MASTODONSAURIDÆ Huxley, 1863.= Huxley, Quart. Jour. Geol. Soc., XIX, p. 65, 1863. Lydekker, R., Cat. Fossil Reptilia Amphibia, pt. IV, p. 141, 1890. Skull triangular, and more or less elongated, with the cranial bones very strongly sculptured, the occipital condyles ossified (49), and large palatal vacuities; dentine of teeth with very complex plications (502); no bony rings (242) in sclerotic; and no ventral scutes. Bodies of vertebræ (49) fully ossified in the adult. There are large palato-vomerine tusks on the inner side of the maxillary teeth; and the palatines run parallel to the maxilla. The mandible has a large postarticular process; and there is a small inner series of mandibular teeth. In the type genus the pubes are separate from the ischia, and do not enter into the formation of the acetabulum; and the sacral ribs form kidney-shaped disks (393). Represented in North America by a single tooth from the Carboniferous of Kansas. Described by Williston as Mastodonsaurus sp. (Kans. Univ. Quart., VI, pp. 209-210, pl. XXI). Represented in the Triassic of Wyoming by Anaschisma browni, Branson (49). =Mastodonsaurus sp. indet.= Williston, Kans. Univ. Quart., VI, p. 209, 1897.