Sub-Class II.—Dipnoi.

Fishes with the skeleton partly cartilaginous, partly osseous; no occipital condyle. Bulbus arteriosus with two longitudinal valves; air-bladder double, lung-like, communicating by a duct and glottis with the hæmal side of the œsophagus, with a pulmonary vein. A narrow gill-opening on each side, with a rudimentary gill-cover; some of the branchial arches without gills; gills free, membranaceous. Nostrils double on each side. Intestine with a spiral valve. Optic nerves not decussating. Oviducts distinct. Ventral fins abdominal.