Fins with spines, dorsal short, anal long .......... 3. Lipogenyidae.
Dorsal fin formed of a series of spines, anal long, formed partly of spines and partly of soft rays .......... 4. Notacanthidae.
B. Vent immediately behind the gill-opening; no caudal fin; scales absent .......... 5. Fierasferidae.
Fam. 1. Dercetidae.—Body much elongate; ordinary scales small or wanting, but two or more continuous series of enlarged scutes along each side; mouth large, praemaxillaries apparently forming the greater part of the upper border of the mouth, which is toothed; opercular apparatus complete. Dorsal fin more or less extended, without spines; anal short, caudal separate; ventrals with not less than 7 or 8 rays.
Dercetis, Leptotrachelus, Leptecodon, Pelargorhynchus, and Stratodus, from the Upper Cretaceous of Europe, Syria, and North America.
Fam. 2. Halosauridae.—Body elongate, covered with cycloid scales, the tail tapering to a point, without caudal fin; head with scales; mouth moderate, bordered by the praemaxillaries and the maxillaries, both toothed; suborbitals large; praeopercle rudimentary. Dorsal fin short, formed of soft rays, above or a little behind the ventrals, which are rather far back, and formed of 9 or 10 rays; anal very long, without spines, extending to the end of the tail. Ovaries transversely laminated, the ova falling into the abdominal cavity. Some 10 living species are known, referred to three genera, inhabiting the Atlantic, Pacific, and Indian Oceans, at depths of 500 to 1400 fathoms.
In Halosaurus the scales of the lateral line, which runs near the lower profile, are scarcely enlarged, and are destitute of luminous organs. Halosaurichthys differs in the union of the ventral fins with each other, as in Notacanthus. In Halosauropsis the scales of the lateral line are strongly enlarged and pouch-like, and bear photophores.
This family is one of great antiquity, being represented in the Upper Cretaceous of Westphalia by Echidnocephalus, which, as shown by A. S. Woodward, appears to have been closely related to Halosaurus.
Fig. 379.—Halosauropsis macrochir, ⅓ nat. size. (After Günther.)