238 FISHES CHAP. 
fins, by a sequence of structural modifications in the same direc- 
tion as in the median fins. The initial stage was probably 
marked by the fusion of the proximal portions of the radialia 
to form a basal support or basipterygium for the free distal 
portions. Subsequently, it may be, a rudiment of the future 
limb-girdle became segmented off from the inner extremity 
of the basipterygium, and by its dorsal and ventral growth 
in the body-wall the lateral half of a girdle was developed. 
The subsequent union of the two halves across the mid-ventral 
line resulted in the evolution of the dorsally incomplete hoop 
of cartilage which is the primary form of thé complete lmb- 
girdle in Craniates. The primitive fin skeleton or “archi- 
pterygium ” was formed from the residue of the basipterygium 
in conjunction with the free distal radialia which it carried. 
The precise structure of the archipterygium is purely hypothetical. 
Possibly it was a biserial fin of the Plewracanthus or Neoceratodus 
type, consisting of a cartilaginous segmented axis, fringed along 
its anterior and posterior, or pre-axial and post-axial margins, by a 
series of slender, simple, or jointed radiala (Fig. 147); or it 
may have been a uniserial structure, somewhat resembling the 
pelvic fin of Plewracanthus, or the pectoral and pelvic fins of 
existing Elasmobranchs (Figs. 250, 141), in which an axis 
formed by the residue of the basipterygium or metapterygium 
had a fringe of radialia on its anterior or preaxial side only. 
If the archipterygium was biserial then the uniserial fin was 
probably derived from it by the subsequent suppression of all 
the post-axial radialia ; or, if uniserial, the biserial fin was evolved 
by a later extension of radialia on to the post-axial margin. The 
evidence of comparative anatomy is not conclusive as to the 
nature of the archipterygium, and palaeontology seems to support 
either view with puzzling impartiality." It may be admitted 
that the lateral fin theory offers the best solution of the problem 
of the origin of the paired fins, but it must be borne in mind 
that no Fish, living or fossil, is known to possess fins of this 
nature, unless the singular lateral lobes of some Ostracodermi 
(e.g. the Coelolepidae) are kindred organs”; neither do continuous 
lateral fins ever exist as vestiges, unless, indeed, the bilateral 
series of spines, which extend between the pectoral and pelvic 
1 Traquair, Nature, 62, 1900, p. 502. 
* Traquair, Z’rans. Roy. Soc. Edin. xxxix. 1899, p. 848. 
