158 FISHES CHAP. 
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 lable to undergo extraordinary 
changes in position, and, as will be seen presently, are much more 
prone to exhibit the effects of adaptive modification and degenera- 
tion. They are present in all existing Fishes, with the exception 
of the Crossopterygian Calamichthys and some Teleosts, and, 
except in the Teleostei, they imvariably 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 le 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). 
Both the median and the paired fins are supported by an 
internal skeleton, consisting (i.) of a series of cartilaginous or 
bony, rod-like radial elements or pterygiophores, for the support 
of the inner or proximal portion of the fins, and (i1.) of a series 
of horny fibres, or bony dermal fin-rays, which fulfil a like 
function for the outer or distal portion. The radial elements, 
however, are never visible externally, even when, as in most 
Elasmobranchs, they support the greater part of the fins, 
inasmuch as they are invested by the fin-muscles and the skin ; 
and in the same group, where horny fibres complete the fin- 
skeleton, they too are’ covered by the spinose skin, and hence 
offer no external evidence of their existence. In the Teleostomi 
a marked reduction in the number and length of the radial 
elements of the paired fins, and the insinking of those pertaining 
to the median fins into the adjacent muscles of the body-wall, 
leaves the dermal fin-rays, with their thin covering of transparent 
and usually scaleless skin, as obvious features in the external appear- 
ance of the Fish, and apparently as the sole support of the fins. 
The dermal fin-rays of the Teleostomi exhibit an obvious dis- 
tinction into spines and soft rays (Fig. 93, A). The former are 
stout, rigid, and unbranched structures, pointed at their free distal 
ends, which, in numbers differing in different genera and species, 
support the anterior portions of the dorsal, anal, and pelvic fins. 
