THE SUBCLASS DIAPSIDA 285 



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 con- 

 stant 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 ac- 

 credited with family rank. 



Family Aetosauridae. Twenty-five presacrals; two sacrals. Hu- 

 merus 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, Eng- 

 land. 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 open- 

 ings large, the orbits relatively small. Upper temporal opening not 

 depressed below level of [parietals]. Palatines approximated or con- 



