VIII MEDIAN FINS 2:35 
the expense of the middle and distal segments. The cause of 
this reduction is often, but not always, to be found in the 
fact that, whenever the dermal fin-rays take the form of stout 
spines, as in the anterior dorsal fin in many Acanthopterygian 
Teleostei, the segmentation of their radialia would obviously 
detract from their value as skeletal supports, and hence they 
rarely consist of more than their proximal segments, although 
the radialia which in the same Fish support soft rays may be 
bi-segmented or tri-segmented. The radialia are, however, un- 
segmented, even slightly branched, cartilaginous rods in the 
Cyclostomata ; short simple rods in the Holocephali ; and equally 
simple bony rods in the dorsal fin of Polypterus, where they sup- 
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i Fre. 137.—The first four radialia 
Fic. 136.—The tri-segmented radialia and the fin- of the dorsal fin of Mesoprion 
rays of part of the dorsal fin of Amia calva. gembra, showing the chain- 
p.s, m.s, and d.s, The proximal, middle, and links for the ring-like bases of 
distal segments of a radial ; fr, fin-rays. (From the fin-rays. 7.¢1, 7.e4, First 
Bridge. ) and fourth proximal radialia. 
port the strong spines of the numerous finlets; but they are bi- 
segmented in the soft-rayed anal fin. As previously mentioned, 
the proportional share taken by the radialia and the horny fibres 
or the dermal fin-rays in the support of the fins differs greatly 
in different Fishes. In the Cyclostomata radialia are the sole, 
and in Elasmobranchs the main supports, and they may extend 
nearly to the free margin of the fin. In the more specialised 
Fishes, as in most Teleostomi, the reverse is the case. The 
radialia sink into the muscles of the body-wall and leave the 
strongly developed fin-rays as the sole support of the visible 
portions of the fins. In not a few Fishes there is an obvious 
segmental correspondence between the radialia and the vertebral 
neural or haemal spines, to the extent that the former equal the 
latter in number and articulate with their distal extremities, as, 
for example, in the caudal region of Plewracanthus and in existing 
