armor, and Euparkeria alone has sclerotic plates; the latter has been accredited with an interparietal bone.
With the inclusion of the doubtful forms there are but few constant characters to distinguish the group from the Rhynchocephalia; typically, however, the absence of palatal teeth, and the attachment of the dorsal ribs are decisive. As a whole, however, the group is one of wide genetic possibilities and [may] have had a close genealogical relationship with all the other members of the Archosauria, and especially the Saurischia. Nearly every known genus has been accredited with family rank.
Family Aetosauridae. Twenty-five presacrals; two sacrals. Humerus a little longer than radius and ulna; hind legs a half longer than the front. Dorsal scutes transversely elongate, covering the whole back; abdomen with small plates.
Triassic. Aetosaurus Fraas, Dyoplax Fraas, Germany. Stegomus Marsh, Connecticut.
Family Ornithosuchidae. Scapula slender, coracoid short and broad. Legs very slender, the epipodials a little longer than the propodials. Two rows of dermal plates, each longer than broad.
Euparkeria is accredited with an interparietal, the only member of the group.
Triassic. Ornithosuchus Newton, ? Erpetosuchus Newton, England. Euparkeria Broom, Sphenosuchus Haughton, South Africa.
Family Scleromochlidae. Premaxillae united. Twenty-one presacrals, three sacrals. Scapulae slender, coracoid long. Tubes long and slender, expanded at extremity; calcaneum with tuberosity; feet as long as tibia, the epipodials longer than propodials. Slender parasternal ribs. No dermal armor.
Triassic. Scleromochlus Woodward, England.
B. Suborder Pelycosimia
Large, heavily built, terrestrial or marsh reptiles. External and internal nares near extremity of triangular skull. Antorbital openings large, the orbits relatively small. Upper temporal opening not depressed below level of [parietals]. Palatines approximated or con-
