Head and body scaly; barbels none. Margin of the upper jaw formed by the intermaxillaries mesially, and by the maxillaries laterally. Opercular apparatus incomplete. Tail prolonged, tapering. Adipose fin none. Dorsal short, belonging to the caudal portion of the vertebral column; anal very long. Stomach without blind sac; two pyloric appendages. Pseudobranchiae none; air-bladder present, divided in the interior. The ova fall into the cavity of the abdomen before exclusion. On each side a parieto-mastoid cavity leading into the interior of the skull.
One genus only (Notopterus) with five species which inhabit fresh waters of the East Indies and West Africa. Well-preserved remains of this genus occur in the marl slates of Padang, in Sumatra. Their air-bladder is divided into several compartments, and terminates in two horns anteriorly and posteriorly, the anterior horns being in direct connection with the auditory organ.
Twenty-seventh Family—Halosauridæ.
Body covered with cycloid scales; head scaly; barbels none. Margin of the upper jaw formed by the intermaxillaries mesially, and by the maxillaries laterally. Opercular apparatus incomplete. Adipose fin none. The short dorsal belongs to the abdominal part of the vertebral column; anal very long. Stomach with a blind sac; intestine short; pyloric appendages in moderate number. Pseudobranchiæ none. Air-bladder large, simple; gill-openings wide. Ovaries closed.
The only genus belonging to this family was discovered by the Madeiran ichthyologist Johnson, in 1863; but since then the naturalists of the “Challenger” expedition have added four other species, showing that this type is a deep-sea form and widely distributed; the specimens were dredged in depths varying from 560 to 2750 fathoms.
Twenty-eighth Family—Hoplopleuridæ.
Body generally with four series of subtriangular scutes, and with intermediate scale-like smaller ones. One (?) dorsal only; head long, with the jaws produced.
Extinct; developed in the chalk and extending into tertiary formations: Dercetis (with the upper jaw longest), Leptotrachelus, Pelargorhynchus, Plinthophorus, Saurorhamphus (with the lower jaw longest), Eurypholis; Ischyrocephalus (?). The latter genus, from cretaceous formations of Westphalia, is said to have two dorsal fins.
Twenty-ninth Family—Gymnotidæ.
Head scaleless; barbels none. Body elongate, eel-shaped. Margin of the upper jaw formed in the middle by the intermaxillaries, and laterally by the maxillaries. Dorsal fin absent or reduced to an adipose strip; caudal generally absent, the tail terminating in a point. Anal fin exceedingly long. Ventrals none. Extremity of the tapering tail capable of being reproduced. Vent situated at, or at a short distance behind, the throat. Humeral arch attached to the skull. Ribs well developed. Gill-openings rather narrow. Air-bladder present, double. Stomach with a cæcal sac and pyloric appendages. Ovaries with oviducts.