. The Cambridge natural history. Zoology. MEDIAN FINS 235 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,
. The Cambridge natural history. Zoology. MEDIAN FINS 235 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, uu- 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-. PiG. 136.—The tri-segmented radialia and tlie fin- rays of part of the dorsal fin of Amia calva. , , and , The proximal, middle, and distal segments of a radial ; , fin-rays. (From Bridge.) Fig. 137.—The first four radialia of the dorsal fin of Mesoprion gembra, showing the chain- links for the ring-like bases of the fin-rays. \ ''. First 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 t
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