Like the median fins, the paired fins may also be regarded as discontinuous remnants of an originally continuous _lateral_ fin which extended along each side of the body from the head to the vent, and of which only the anterior and posterior portions now remain as the pectoral and pelvic fins. Pectoral fins are rarely absent in existing Fishes, and when present they are always situated just behind the branchial clefts, where, as in most Teleostomi, the outline of their supporting pectoral girdle can often be seen. They vary greatly in form and size in different Fishes, and in the Elasmobranchs are larger than in most others. In certain members of the latter group, the Skates and Rays, in which the feebly-developed tail is probably useless as a locomotor organ, the pectoral fins are exceptionally large, forming broad triangular lobes, the broad bases of which are continuous with the sides of the body from the anterior part of the head to near the origin of the pelvic fins, and thus in outward form, if not in inward structure, simulate re-acquired continuous lateral fins. Except in a few instances, the Teleostomi have relatively small fan-shaped or paddle-like pectoral fins, and usually only that portion of each fin which is supported by the dermal fin-rays is visible externally. In the Crossopterygii, however, each fin appears to consist of a central lobe invested by scales and encircled by a peripheral fringe of fin-rays, and is hence described as a "lobate" fin (Fig. 279). When the central lobe is much increased in length but reduced in width the fin becomes acutely lobate. A similar type of fin is present in the Dipnoi, but in _Protopterus_ and _Lepidosiren_, owing to the length and narrowness of the central lobe, and the reduction or suppression of the marginal fringe, the pectoral members assume the condition of long tapering filaments (Fig. 304). {158}Although as a rule smaller in size, the pelvic fins bear a general resemblance to the pectoral fins, but in certain groups, especially in Teleosts, they are liable to undergo extraordinary changes in position, and, as will be seen presently, are much more prone to exhibit the effects of adaptive modification and degeneration. They are present in all existing Fishes, with the exception of the Crossopterygian _Calamichthys_ and some Teleosts, and, except in the Teleostei, they invariably retain their primitive position near the junction of the trunk with the tail, and directly in front of the cloacal or the anal aperture; in this position they are said to be "abdominal." In other Teleostei the fins undergo forward displacement and come to lie directly beneath the pectorals (Fig. 415), when they are said to be "thoracic," as in the Mackerels (Scombridae) and the Horse-Mackerels (Carangidae); or even in front of the pectoral fins on the under surface of the throat, when they are described as "jugular," as in the Cod and other Gadidae (Fig. 398).