The body is usually elongate and slender, ending in a pointed tail, the head large and heavy, and the eyes, in the genera confined to the abysmal zone, usually so far degenerated that they are not visible at all from the outside. The Macruridæ form a family that contributes very largely to the deep-sea fauna; no fewer than twenty-six different species are known to occur within the limits of the abysmal zone. Not only do the Macruridæ contribute a large number of different species, but they probably occur, in some districts at any rate, in vast numbers. During the voyage of the ‘Talisman,’ for example, the French naturalists caught in one haul of the dredge off the coast of Morocco in 500 fathoms of water no fewer than 134 fish, of which number 95 belonged to the family Macruridæ. They are usually small fish, measuring from a few inches to two feet in length, with a body terminating in a long compressed tapering tail and covered with spiny, keeled, or striated scales. The Pleuronectidæ or flat fish are not, as a rule, found in the abysmal zone; one species, however, _Pleuronectes cynoglossus_, was found by the American ship ‘Blake’ to extend into 732 fathoms of water. The families Sternoptychidæ and Scopelidæ are of particular interest to us, as almost all the genera they contain belong either to the pelagic or abysmal zones, and lend support to the view enunciated by Moseley, that the deep-sea fauna has, partly at any rate, been derived from the fauna of the pelagic zone. They are nearly all small slender fish with delicate and frequently semi-transparent bodies, large gaping mouths armed with numerous long irregular teeth, and frequently provided upon the head and sides of the trunk with rows of eye-like phosphorescent organs. These families, and others that have still to be referred to, belong to the group of Teleostei that is called Physostomi, the name referring to the open communication that usually exists in all these families between the swimming bladder and the alimentary canal. It is a remarkable fact that in none of the deep-sea representatives has this open communication been discovered. It is true that many specimens are, when examined, so lacerated by the diminution in pressure as to render anatomical study a matter of difficulty, but still a fair number of uninjured well-preserved specimens have now been examined and the duct has not been found. Of the family Sternoptychidæ, _Gonostoma microdon_ has a most remarkable distribution. It has been found at numerous stations in both the Pacific and Atlantic Oceans at depths ranging from 500 to nearly 3,000 fathoms of water.