VI EXTERNAL CHARACTERS Lay, 
dorsal fins (Fig. 398), or even into a series of isolated finlets; and 
similarly also with the 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 Jateral tin 
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 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 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). 
