Lydekker, Cat. Fossil Reptilia and Amphibia, IV, p. 201, 1890.

The following characterizations of the family are those given by Lydekker ([393, p. 201]) based on Fritsch ([251]): Body slender and lizard-like; skull narrow, with smooth or faintly sculptured bones; neural spines of vertebræ well developed, and long, slender ribs. Teeth smooth, or with grooved summits. The whole body covered with sculptured scutes. Internal gills may be developed.

Fritsch ([251, Bd. I, p. 159]) gives the following in his original description:

"Stegocephali von Baue schlanker Eidechsen mit schlanken langen Rippen. Wirbel amphicoel mit stark entwickelten oberen Dornfortsätzen. Schädelknochen glatt oder schwach verziert. Schuppen gross, verziert, den ganzen Körper deckend. Zähne glatt oder mit verzierte Spitze. Kiemenbogen bei einigen angedeutet. Mittlere Kehlbrustplatte unbekannt. Coracoidea ähnlich wie bei Branchiosaurus schlank, winkelig gebogen."

Fritsch includes the following genera in the family:

Hylonomus, Hyloplesion, Smilerpeton, Seeleya, Orthocosta, and Ricnodon. The family includes the following species:

Hylonomus lyelli Dawson, Nova Scotia.
latidens Dawson, Nova Scotia.
multidens Dawson, Nova Scotia.
wymani Dawson, Nova Scotia.
fritschii Gein. and Deichm., Saxony.
geinitzi Credner, Saxony.
(?) pictus Fritsch, Bohemia.
wildi Woodward, England.
Smilerpeton aciedentatum Dawson, Nova Scotia.
Hylerpeton dawsonii Owen, Nova Scotia.
intermedium Dawson, Nova Scotia.
longidentatum Dawson, Nova Scotia.
Fritschia curtidentata Dawson, Nova Scotia.

Genus HYLONOMUS Dawson.

Dawson, Quart. Jour. Geol. Soc. London, XVI, p. 274, figs. 14-18, 1860

Dawson, Air-breathers of The Coal Period, p. 44, 1863.