VI EXTERNAL CHARACTERS I 5 7 



dorsal fins (Fig. 398), or even into a series of isolated finlets ; and 

 similarly also with tlie ventral portion or anal fin ; or, without 

 undergoing subdivision, both fins may become reduced in length 

 to an extent which differs greatly in different Fishes, and persist as 

 single dorsal or anal fins. But even when a median fin is reduced 

 in length by atrophy, or becomes subdivided by breaches in its 

 continuity, the externally invisible supporting radial elements 

 frequently remain to prove the originally greater length of the 

 fin, or the continuity of its detached remnants. 



Like the median fins, the paired fins may also be regarded 

 as discontinuous remnants of an originally continuous laterctl 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, tlie outhne 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 Eays, 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 tl^at portion of each 

 fin which is supported by the dermal fin -rays is visible exter- 

 nally. In the Crossopterygii, however, each fin appears to con- 

 sist 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 Frotojpterus 

 and Lepiclosiren, 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). 



