THE OCEAN SUNFISHES (FAMILY MOLIDAE) By A. FRASER-BRUNNER SYNOPSIS The relationships of the Molidae with other Plectognathi are briefly discussed. The movable lobe at the hind margin of the body, supported usually by migrant dorsal and anal rays but sometimes also by caudal rays centrally, is designated the ' clavus '. Three genera are recognized, assigned to two subfamilies. Masturus is shown to include two forms (treated as species but possibly the sexes of one). Evidence is presented to show that in this genus alone of the family some caudal rays are developed. Mola is shown to include two species, which are diagnosed and figured. Sexual dimorphism in Mola mola is described. Full synonymies are included. ON account of their curious form and the great size which they often attain, the fishes of the family Molidae, usually called Ocean Sunfishes, have attracted considerable attention from early times. A large and scattered literature exists concerning them, but although comparative studies have been made from time to time and their anatomy has received attention quite frequently, we are still far from a complete understanding of their relationships. This is mainly because all the species are rather rare, and their occurrence unpredictable, so that it is not possible to make an excur-sion for the express purpose of collecting specimens, as could be done with many other fishes, while the great size of most examples makes transportation and preservation a difficult problem. Consequently good comparative material is not easily available for study, and much reliance has to be placed upon published descriptions and figures. It is the purpose of the present work to draw attention to certain facts which have become apparent from a study of the literature, aided by the material in the national collection. My thanks are due to Mr. G. Palmer for his assistance in seeking out some of the references and checking a number of points in them. I am concerned here only with taxonomy within the family, since a full considera-tion of their relationship to other Plectognathous fishes will be included in a larger work upon the anatomy and phylogeny of the whole Order now in preparation. It can be pointed out here, however, that I have already indicated in an earlier paper (Fraser-Brunner, 1943), that the Molidae are not really as highly specialized as previously supposed. Their main peculiarity lies in the atrophy of the rear end of the vertebral column, resulting in a mechanical rearrangement of the median fin-structures closely resembling that seen in other fishes when the tail is amputated al an early age ; some interesting examples of this among Flatfishes have been given by Chabanaud (1935). The resemblance is not quite perfect, since with amputation the supporting bones of dorsal and anal fins are lost with the tail, whereas in the Molidae only the vertebral structures are lost. The later alis muscles of the trunk, deprived of their normal attachment, become inserted upon the deep muscles of the dorsal and anal fins, and progressively lose their identity in the genera Ranzania, Masturus, Mola. The result of this is that body-flexion is lost but the dorsal and anal fins gain in power, and the latter are therefore
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