XXIII ACANTHOPTERYGII 679 
seas, Newfoundland, California, and Japan. It supplies im- 
portant fisheries in France and Italy. The Tunnies are the only 
fish known to be warm-blooded. 
Fam. 4. Trichiuridae.—Praemaxillaries not protractile. 
Vertebrae 32 to 160, without transverse processes; ribs sessile, 
on the centra or on the haemal arches when these are present ; 
epipleurals, if well developed, on the centra. Scales very small 
or absent. Spinous portion of dorsal fin much longer than the 
soft, the spines more or less feeble. Pectoral fins inserted low 
down the sides. Pseudobranchiae present. 
The members of this family show, a great variation in the 
shape of the body, which, although always strongly compressed, 
is not unlike that of a Mackerel in the more normal types, such 
as Thyrsites and Ruvettus, whilst, through a chain of genera, it 
generally assumes an extremely elongate form ; concurrently with 
this elongation of the body, the dorsal fin loses its differentiation 
into two portions, the ventrals become reduced and disappear, as 
in the Seabbard- or Frost-Fish (Lepidopus caudatus), while the 
caudal fin decreases in size, loses its fork-shape, and is finally 
lost in Zriehiurus, in which the body is ribbon-shaped and tapers 
to a point. 
About 25 species are known, pelagic and widely distributed, 
many descending to great depths. 
Principal living genera: Ruvettus, Thyrsites, Epinnula, Nesi- 
archus, Nealotus, Promethichthys, Dicrotus, Gempylus, Aphanopus, 
Lepidopus,  Euoxymetopon, Benthodesmus,  HEupleurogrammus, 
Trichiurus. 
Remains of several species referred to Zhyrsites, Lepidopus, 
and to the extinet genera Thyrsitocephalus, Hemithyrsites, and 
Trichiurichthys, have been found in the Oligocene and Miocene of 
Europe. 
Fam. 5. Histiophoridae.—Praemaxillaries not protractile ; 
snout produced into a spear-shaped rostrum ; a praedentary bone ; 
teeth minute. Body elongate, covered with small or rudimentary 
scales. Vertebrae 24 or 25, without transverse processes; ribs 
sessile; no epipleurals. One or two dorsal fins, without a 
distinctly spinous portion. Pectoral fin low down the side. 
Pseudobranchiae present. 
The Sail-Fishes are large oceanic fishes, endowed with great 
strength and swiftness, occurring in the tropical and sub-tropical 
