2306 
FISHES CHAP. 
Dipnoi: 
In others again, as in most Teleostomi, there is no such 
segmental relation, and the radiaha are more numerous than the 
Fic. 138.—The posterior dorsal 
fin of Holoptychius lepto- 
pterus from the old Red Sand- 
stone of Nairnshire. Traces 
of dermal fin-rays may be 
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, Holocephah, 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- 
Besaaroezes 
alia and fin- Hy 
; Hy 
rays is very H4 
rare, and has Hi 
only been ob- 
served in the 
rag fg.3 
seen at the distal margin of caudal r eg10n po 2 
the fin. (After Smith Wood- of eertain 
ward. ) x 56 
Crossopteryg 11 
(e.g. the Coelacanthidae).' 
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- 
ous lateral fins. The concentration of 
radiala 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. 
Fria. 139.—A dermal fin-ray and 
its supporting radial or 
pterygiophore in the Trout 
(Salmo fario). D.F’.R, Dermal 
fin-ray ; PTG.1, PTG.2, pég.3, 
the proximal, middle, and 
distal segments of which the 
tri-segmented radial consists ; 
ptg.3 is cartilaginous ; the 
other two are bony. (From 
Parker and Haswell.) 
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 Plewracanthus, and especially in the 
1 Smith Woodward, Nat. 
SCR me ISK ya 748). 
