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These are deep, thin, flat-sided little fishes, with various spiny projections, large oblique mouths with small teeth, large eyes which are directed upwards in some of them but sidewise in others, and ventral fins placed far back. Some of them have an adipose fin behind the rayed dorsal, but others do not. All of them are silvery, and all of them have series of large and conspicuous luminescent organs on the lower part of the body. They are to be found in the mid-depths in all oceans, sometimes in great abundance.