238 FISHES CHAP. 



fins, by a sequence of structural modifications in the same direc- 

 tion as in the median tins. 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 the complete limb- 

 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 Pleuracanthus 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 radialia (Fig. 147); or it 

 may have been a uniserial structure, somewhat resembling the 

 pelvic fin of Pleuracanthus, 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. 1 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. 

 2 Traquair, Trans. Roy. Soc. Edin. xxxix. 1899, p. 843. 



