Lizard-like, sometimes longicaudate, stegocephalous, lepospondylous, ambulatory or legless amphibians; skull bones usually sculptured with pits and grooves; lateral-line canals well developed on skull bones; skull with horns from tabulare and supratemporals or without horns; branchiæ never persistent; sclerotic plates present; orbits usually well forward. Vertebræ hour-glass-shaped; endochondral bone weakly developed throughout skeleton, especially in vertebræ; notochord largely persistent; neural spines low and rudimentary or long, fan-shaped, and highly ornamented. Dorsal series of vertebral column variable; usually from 22 to 30; tail containing sometimes over 75 vertebræ, or tail very short with 2 weakly developed vertebræ; caudal ribs present, in those forms with long caudal series the distal vertebræ sometimes exhibiting 2 pleurocentra. Ribs long and curved, always intercentral in position, single-headed, with at times an incipient tubercle. Pectoral girdle composed of scapulæ, clavicles, coracoids, and interclavicle. Pelvic girdle composed of osseous rod-like ilium, plate-like ischium; pubis cartilaginous, sometimes calcined. Limbs present or wanting or weakly developed, sometimes present in front and wanting behind. Radius and ulna and tibia and fibula free; carpus and tarsus usually cartilaginous; digits 4 in hand and 5 in foot, terminal phalanges sometimes clawed. Phalangeal formula for the hand 2-2-3-2, for the foot 2-2-3-4-3. Abdomen covered with dermal armature composed of osseous or corneous rods or scutes; overlapping scales, fish-like in appearance, sometimes present over the entire body; body also covered with lizard-like scales or naked. The order Microsauria was established by Sir William Dawson in 1863 (208) as a _family_ of "reptiles" for the reception of the genera _Hylonomus_, _Hylerpeton_, _Smilerpeton_, and _Fritschia_. _Hylonomus lyelli_ is the type species of the order. Dawson (216, p. 635) says of the species _Hylonomus lyelli_ Dawson: "It is the type of the genus _Hylonomus_ and of the family Microsauria." The forms which have been referred to the genus _Hylonomus_, and hence to the order Microsauria, from the deposits of Europe are discussed under _Hylonomus_.