. The Cambridge natural history. Zoology. 236 FISHES. Dipnoi. Ill others again, as in most Teleostomi, there is no such segmental relation, and the radialia are more numerous than tlie vertebrae whenever the two are co-ex- tensive. The exoskeletal fin-supports exhibit similar relations to their radialia, but in inverse order. Much inore numer- ous than the radialia in the Elasmo- branchs, Holocephali, and the Dipnoi, the former become gradually reduced in the Teleostomi,until in the Holostei and Tele- ostei they correspond in number with the supporting radialia. Complete numerical corresponden
. The Cambridge natural history. Zoology. 236 FISHES. Dipnoi. Ill others again, as in most Teleostomi, there is no such segmental relation, and the radialia are more numerous than tlie vertebrae whenever the two are co-ex- tensive. The exoskeletal fin-supports exhibit similar relations to their radialia, but in inverse order. Much inore numer- ous than the radialia in the Elasmo- branchs, Holocephali, and the Dipnoi, the former become gradually reduced in the Teleostomi,until in the Holostei and Tele- ostei they correspond in number with the supporting radialia. Complete numerical correspondence between the neural and haemal spines and the radi- alia and fin- rays is very Fig. 138.—The posterior rare, and has fin of Holoptychius hpto- , v i ^teriM from the old Red Sand- "^^V "««" "'' stone of Nairnshire. Traces served in the of dermal fin-rays may be i -. seea at the distal margin of Cauaal region the fin. (After Smith Wood- of certain ward.) /-I i • • Crossopterygii ( the Coelacanthidae).^ In not a few Fishes the radialia of the median fins undergo modifications which oifer an interesting parallel to an early stage in the evolution of the paired fins from primitively continu- ous lateral fins. The concentration of radialia which occurs in isolated median fins often results, through growth pres- sure, in the complete fusion of the proximal segments of more or fewer of the radialia into two or three basal supports, or even into a single basal piece. Examples of such basal fusion are frequent in the dorsal fins of Elasmobranchs, and the same modification may also be seen in the anal fin of Pleuracanthus, and especially in the 1 Smith Woodward, Nat. Sc. i. 1892, p. 29. Fig. 139.—A dermal fin-ray and its supporting radial or pterygiophore in the Ti-out (Salmofario). i>,i^.7J, Dermal fin-ray; , 7',, the proximal, middle, and distal segments of which the tri-segmented radial consists ; ptg.^ is cartilaginous ; the
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