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Rh. Permocarboniferous (Uppermost Carboniferous and Lowermost Permian).

(Pelycosauria). Carnivorous reptiles of from four to eight feet in length, with long, often very long, dorsal spines; three sacral vertebrae.

Sphenacodon Marsh, New Mexico. Dimetrodon Cope, Texas. Clepsydrops Cope, Illinois, Texas. Tetraceratops Matthew, Texas. Bathygnathus Leidy, Prince Edward Island.

(Edaphosauria). Subaquatic or terrestrial invertebrate feeding reptiles, from six to eight feet in length. Spines of dorsal vertebrae very long, each with transverse processes. Skull small, short, high, with numerous palatal and coronoid conical teeth.

Edaphosaurus Cope, Texas, New Mexico. Naosaurus Cope, Texas, New Mexico, Ohio, Germany, Russia.

. Lizard-like, insectivorous, four or five feet in length. Teeth conical; spines of vertebrae short; two sacral vertebrae. Texas and New Mexico.

Varanops Williston, Varanosaurus Broili, Poliosaurus Case, Poecilospondylus Case, Arribasaurus Williston, Scoliomus Williston and Case.

. About six feet in length, carnivorous. Skull narrow; teeth slender and conical or flattened; temporal opening small, an upper one also in Ophiacodon; ribs holocephalous; limbs short and stout; two sacral vertebrae. Texas and New Mexico.

Ophiacodon Marsh, Theropleura Cope, Diopeus Cope, Secodontosaurus Williston.

(Caseasauria). Thickset, crawling and probably burrowing, invertebrate-feeding reptiles about four feet long. Skull