=Scabbard-and Cutlass-fishes: Lepidopidæ and Trichiuridæ.=—The family of _Lepidopidæ_, or scabbard-fishes, includes degenerate mackerels, band-shaped, with continuous dorsal fin, and the long jaws armed with very small teeth. These are found in the open sea, _Lepidopus candatus_ being the most common. This species reaches a length of five or six feet and comes to different coasts occasionally to deposit its spawn. It lives in warm water and is at once chilled by the least cold; hence the name of frostfish occasionally applied to it. Several species of _Lepidopus_ are fossil in the later Tertiary. _Lepidopus glarisianus_ occurs in the Swiss Oligocene, and with it _Thyrsitocephalus alpinus_, which approaches more nearly to the _Gempylidæ_. Still more degenerate are the _Trichiuridæ_, or cutlass-fishes, in which the caudal fin is wanting, the tail ending in a hair-like filament. The species are bright silvery in color, very slender, and very voracious, reaching a length of three to five feet. _Trichiurus lepturus_ is rather common on our Atlantic coast. The names hairfish and silver-eel, among others, are often given to it. _Trichiurus japonicas_, a very similar species, is common in Japan, and other species inhabit the tropical seas. _Trichiurichthys_, a fossil genus with well-developed scales, precedes _Trichiurus_ in the Miocene. [Illustration: FIG. 209.—Cutlass-fish, _Trichiurus lepturus_ Linnæus. St. Augustine, Fla. ] =The Palæorhynchidæ.=—The extinct family of _Palæorhynchidæ_ is found from the Eocene to the Oligocene. It contains very long and slender fishes, with long jaws and small teeth, the dorsal fin long and continuous. The species resembles the _Escolar_ on the one hand and the sailfishes on the other, and they may prove to be ancestral to the _Istiophoridæ_. _Hemirhynchus deshayesi_ with the upper jaw twice as long as the lower, sword-like, occurs in the Eocene at Paris; _Palæorhynchum glarisianum_, with the jaws both elongate, the lower longest, is in the Oligocene of Glarus. Several other species of both genera are recorded. [Illustration: FIG. 210.—_Palæorhynchus glarisianus_ Blainville. Oligocene. (After Woodward.) ]