Fam. 5. Malthidae.—Mouth rather small, subterminal or inferior, with villiform or cardiform teeth. Gill-opening above pectoral; pseudobranchiae absent. Pectoral fin forming an elbow-like angle, with three pterygials. Ventral with 5 rays. Spinous dorsal absent, or reduced to a more or less developed tentacle lodged in a cavity under the snout. Head and body with bony tubercles or spines.
About 30 species are known, mostly from the deep sea within the tropics (down to 1270 fathoms). Principal genera: Coelophrys, Malthe, Malthopsis, Halieutaea, Halicmethes, Dibranchus.
The "Bat-Fish" (Malthe vespertilio), common in shallow water about the West Indies, is said to assume an almost toad-like attitude on the ground, the head being directed slightly upwards, while the pectorals take on the function of hind legs and the ventrals of fore legs.
Sub-Order 13. Plectognathi.
Air-bladder without open duct. Opercular bones more or less reduced; supraoccipital in contact with the frontals, separating the parietals; maxillary and praemaxillary bones often firmly united. Pectoral arch suspended from the skull. No ribs. Ventral fins thoracic and much reduced if present; the pelvic bones, if present, more or less completely co-ossified. Gill-opening much reduced. Body covered with more or less osseous scales, bony scutes, or spines, or naked.
A highly aberrant group, closely connected with the Acanthopterygii through the Acanthuridae, as pointed out long ago by Dareste.[[758]] The skeleton is often feebly ossified and the vertebrae much reduced in number, but the jaws, although short, are very strong, usually with large sectorial teeth which may be confluent into a beak; the post-temporal is short and simple, suturally united to the squamosal. These fishes have usually been arranged in three divisions: Sclerodermi, Ostracodermi, and Gymnodontes, but Regan,[[759]] whose classification is here followed, has shown that the latter include a type (Triodon) which, in spite of its beak-like teeth, is more nearly related to the Sclerodermi, whilst the Ostracodermi have much more in common with the latter than with the Gymnodontes. It therefore appears best to admit only two divisions, the first with 4, the second with 3 families:—
I. Sclerodermi.—Supraclavicle vertical; pectoral arch of the Perciform type; all the vertebrae with a single neural spine.
A. Body covered with hard or spinous scales; epipleurals present; pelvis present.
Teeth separate; spinous dorsal present; ventrals paired; pelvis immovable .......... 1. Triacanthidae. A beak; spinous dorsal and ventrals absent; pelvis movable .......... 2. Triodontidae.
Teeth separate; spinous dorsal present; ventrals absent or represented by a single short spine; pelvis movable .......... 3. Balistidae.