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 dor.sal 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 

 (e.g. 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 Pleur acanthus, 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; PTG.l, 7'TG.2,plg.3, 

 the proximal, middle, and 

 distal segments of which the 

 tri-segmented radial consists ; 

 ptg.^ is cartilaginous ; the 

 other two are bony. (From 

 Parker and Haswell.) 



