Fig. 145.—A, Pectoral fin, and B, pelvic fin of Cladoselache. (From Bashford Dean.)
Fig. 146.—Pectoral fins of various Fishes. A, Acanthias vulgaris; B, Raia sp.; C, Chimaera monstrosa; D, Acipenser rhynchaeus; E, Amia calva; F, Lepidosteus platyrhynchus; G, Polypterus bichir; H, Salmo salvelinus. The preaxial side of each fin is to the left and the postaxial to the right. f.r, Dermal fin-ray; ms, mesopterygium; mt, metapterygium; p, propterygium; r, free radialia; 1, 5, the preaxial and postaxial basal elements in a Teleost, which may be mesopterygial and metapterygial pieces respectively, the three remaining basal pieces probably being intrusive metapterygial radialia directly articulating with the pectoral girdle. In B, D, E, and F, similar intrusive radialia are shown. (From Gegenbaur.)
The broadly lobate pectoral fin of the existing Crossopterygii (Fig. 146, G) is uniserial, closely resembling that of the more typical Elasmobranchs.[[219]] There are three basal elements, a propterygium, a mesopterygium, and a metapterygium, each of which supports a series of partially ossified radialia. Little is known of the endoskeletal elements of the broadly or acutely lobate fins of the fossil Crossopterygii, but it seems probable that their disposition was uniserial and abbreviate in obtusely lobate fins and biserial in acutely lobate fins. In the remaining Teleostomi (Actinopterygii) the endoskeletal elements become gradually reduced in number and importance, their place as fin-supports being usurped by the dermal fin-rays. In addition, more than three, usually several, basal elements articulate directly with the pectoral girdle, and hence the fins become multi-basal. In the Chondrostei and the Holostei a metapterygium is always recognisable, supporting several radialia along its preaxial border, as in Acipenser (Fig. 146, D) and Amia (Fig. 146, E), or only a single one, as in Lepidosteus (Fig. 146, F). The anterior part of the fin is supported by a variable number of cartilaginous or bony radialia, which, with the metapterygium, articulate with the limb-girdle. In Teleosts the process of reduction reaches its maximum. Usually there is but a single row of short, hour-glass-shaped ossicles, of which the postaxial one may represent a vestigial metapterygium, and sometimes there is also a distal row of small cartilages or ossicles, partially hidden in the cleft bases of the dermal fin-rays (Fig. 146, H). In all these Fishes the fin is a much reduced uniserial fin, in which more or fewer of the preaxial radialia have acquired a direct secondary connexion with the pectoral girdle. Of living Dipnoids Neoceratodus has a nearly typical biserial fin, but, as seems to be the case in all fins of this type at present known, there is a marked absence of symmetry in the number and disposition of the radialia on the two sides of the axis. There is also much individual variation. No two fins are precisely alike, and the radialia may sometimes divide. In the very acutely lobate fins of the remaining Dipnoids it is evident that great reduction has taken place. Protopterus has lost all trace of postaxial radialia, and in Lepidosiren even the preaxial have atrophied, leaving only the long jointed axis to represent the originally biserial fin.
Fig. 147.—The left pectoral fin of Neoceratodus. a, b, First two segments of the axis; FS, preaxial horny fibres; †, †, pre- and post-axial radialia. (After Wiedersheim.)
The Pelvic Fins.—In the simplicity of their endoskeletal supports the pelvic fins of Cladoselache are the most primitive type of paired fins at present known (Fig. 145, B). In general structure they resemble the pectorals, but the radialia are fewer in number, less modified by concentration, and exhibit little, if any, trace of basal fusion. Add to such features as these the apparent absence of any trace of pelvic rudiments, or of basipterygia, and it will be obvious that the pelvic fins differ but little from the median fins of the same Fish except that they are paired. In Pleuracanthus the pelvic fins differ from the corresponding pectorals in being uniserial instead of biserial (Fig. 250). All other Elasmobranchs, including the Holocephali, have uniserial fins, which consist of a large metapterygium, supporting a preaxial fringe of segmented radialia. A propterygium is sometimes present, notably in some of the Skates and Rays, and, like the metapterygium, it is directly connected with the pelvic girdle.
Fig. 148.—Skeleton of a pelvic fin of Polyodon folium, ventral view, with the anterior margin of the fin to the right; to show the partial fusion of the proximal portions of primitively distinct radialia to form a basipterygium. b, Inner or mesial extremity of the basipterygium; d.p, dorsally directed, rudimentary iliac process; n, foramen for nerves. (After Rautenfeld.)
The skeleton of the pelvic fins of the Teleostomi is often extremely degenerate. It is perhaps best developed in the Chondrostei,[[220]] where each fin is supported by numerous segmented radialia, more or fewer of which fuse towards the base of the fin, and those form a large and slightly ossified basipterygium (Fig. 148). In the living Crossopterygii, Holostei, and Teleostei, the pelvic fins are similar in essential structure, but are very degenerate. The basipterygium is usually well developed and is always bony (Fig. 149), and in many Teleosts it acquires so extensive a sutural connexion with its fellow that, physiologically, it supplies the place of a true pelvic girdle. At its distal end there may be a single row of small cartilaginous or bony nodules, representing vestigial radialia, as in the Crossopterygii, Holostei, and Teleostei, but even these may be absent, and the dermal fin-rays then articulate directly with the basipterygium. Little is known of the skeleton of the pelvic fins in the fossil Crossopterygii, but there is evidence of the existence of a higher grade of structure than in their surviving allies. In Eusthenopteron,[[221]] for example, the fin is supported by an axis of at least three bony segments, with at least three ossified preaxial radialia; hence, it has obviously undergone less degeneration than in Polypterus, where the fin-skeleton is essentially Teleostean. In the Dipnoi the pelvic fins are similar to the corresponding pectoral fins, but individual variation is more marked and even the central axis may divide.[[222]] In the males of all existing Elasmobranchs, including the Holocephali, certain of the more distally situated metapterygial radialia become modified to form a supporting skeleton for the copulatory organs, the claspers, or mixipterygia. In the latter group the anterior claspers are also provided with cartilaginous supports articulating with the pelvic girdle directly in front of the pelvic fins.