=Rondeletiidæ.=—The _Rondeletiidæ_ are naked black fishes with small eyes, without adipose fin and without luminous spots, taken at great depths in the Atlantic. The relationship of these fishes is wholly uncertain. [Illustration: FIG. 91.—_Cetomimus gillii_ Goode & Bean. Gulf Stream. ] The _Cetomimidæ_ are near allies of the _Rondeletiidæ_, having the mouth excessively large, with the peculiar form seen in the right whales, which these little fishes curiously resemble. [Illustration: FIG. 92.—Headlight Fish, _Diaphus lucidus_ Goode & Bean. Gulf Stream. ] =Myctophidæ.=—The large family of _Myctophidæ_, or lantern-fishes, is made up of small fishes allied to the _Aulopidæ_, but with the body covered with luminous dots, highly specialized and symmetrically arranged. Most of them belong to the deep sea, but others come to the surface in the night or during storms when the sunlight is absent. Through this habit they are often thrown by the waves on the decks of small vessels. Largely from Danish merchant-vessels, Dr. Lütken has obtained the unrivaled collection of these sea-waifs preserved in the Museum of the University of Copenhagen. The species are all small in size and feeble in structure, the prey of the larger fishes of the depths, from which their lantern-like spots and large eyes help them to escape. The numerous species are now ranged in about fifteen genera, although earlier writers placed them all in a single genus _Myctophum_ (_Scopelus_). [Illustration: FIG. 93.—Lantern-fish, _Myctophum opalinum_ Goode & Bean. Gulf Stream. ] In the genus _Diaphus_ (_Æthoprora_) there is a large luminous gland on the end of the short snout, like the headlight of an engine. In _Dasyscopelus_ the scales are spinescent, but in most of the genera, as in _Myctophum_, the scales are cycloid and caducous, falling at the touch. In _Diaphus_ the luminous spots are crossed by a septum giving them the form of the Greek letter θ (theta). One of the commonest species is _Myctophum humboldti_. [Illustration: FIG. 94.—Lantern-fish, _Ceratoscopelus madeirensis_ (Lowe). Gulf Stream. ]