236 



FISHES 



CHAP. 



Dipnoi. In others again, as in most Teleostoini, there is no such 

 segmental relation, and the radialia are more numerous than the 



vertebrae whenever the two are co-ex- 

 tensive. The exoskeletal fin-supports 

 exhibit similar relations to their radialia, 

 but in inverse order. Much more 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 dorsal rare, and has 

 fin of Holoptychius hpto- -, , , 



pterus from the old Red Sand- C ' n V ' n . 



stone of Nairnshire. Traces served in the 

 of dermal fin -rays may be -, , 



seen at the distal margin of Caudal region 



the fin. ( After Smith Wood- of certain 

 ward.) 



Crossopterygn 

 (e.g. the Coelacanthidae). 1 



In not a few Fishes the radialia of 

 the median fins undergo modifications 

 which offer an interesting parallel to 

 an early stage in the evolution of the 

 paired fins from primitively continu- FlG - 189. A dermal fm-ray and 



J; its supporting radial or 



OUS lateral fins. The concentration OI pterygiophore in the Trout 



PTG.1- 



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 PI eur acanthus, and especially in the 



1 Smith Woodward, Nat. Sc. i. 1892, p. 29. 



the proximal, middle, and 

 distal segments of which the 

 tri-segmented radial consists ; 

 ptg.% is cartilaginous ; the 

 other two are bony. (From 

 Parker and Has well.) 



