0am PELVIC FINS 245 
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 
(Pig. 250). <All other 
Elasmobranchs, including 
the Holocephali, have uni- 
serial fins, which consist 
of a large metapterygium, 
supporting a _ preaxial 
fringe of segmented 
radiaha. A proptery- 
gium is sometimes present, 
notably in some of the 
folium, ventral view, with the anterior margin 
1 a =. 4 “: 
Skates and Rays, and, like of the fin to the right; to show the partial 
: oe. fusion of the proximal portions of primitively 
ry 5 AA aie c : 
the metapterygium, 1b 1s distinct radialia to form a basipterygium. 3, 
directly connected with Inner or mesial extremity of the basipterygium ; 
tl elvic. sirdl d.p, dorsally directed, rudimentary iliac process ; 
1e peivic girdle. 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,) 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, repre- 
senting vestigial radialia, as in the Crossopterygii, Holostei, and 
1 Thacker, Trans. Connecticut Acad. iv. 1877, p. 233. 
