ORIGIN OF LIMBS. 57 



Elasraobranchs they are arranged in a fan-like manner, and the 

 fin may be described as rhipidostichous. In Dipnoi the somactids 

 of the fin are represented by a basal piece, followed by a row of 

 them occupying the axis of the fin, with or without pre- and post- 

 axial pieces placed like the barbs of a feather (Fig. 138). Such 

 an arrangement may be termed rachiostichous and mesorachic. 



In some sharks and in the extinct Pleiiracanthidae there is a succession 

 of somactids forming a rachis, but the rachis is placed on one side of the 

 fin and carries peripheral somactids mainly on that side.* Such an arrange- 

 ment may be called rachiostichous and pleurorachic (Fig. 76). 



A similar reduction in the number of basal somactids is sometimes 

 found in the median fins of extinct fishes, e.g. in the anal fins of Pleura- 

 canthus (Fig. 87). 



In the discussion of the vexed question of the origin of the 

 vertebrate paired limbs, much attention is paid to the arrange- 

 ment of these somactids (radialia). According to Gegenbaur 

 the Ceratodus type (Fig. 138) of fin skeleton (rachiostichous) is 

 the most primitive, and this fin constitutes what he calls the 

 archipterygium. On this view the skeleton of the paired fins 

 and their girdles have originated from a branchial arch and its 

 branchial rays ; the girdle being derived from the branchial arch 

 and the somactids from the branchial rays. One important 

 objection (among others) to this view is that the branchial arches 

 are in the gut-wall, whereas the limb girdles lie in the body wall. 



On another and perhaps more acceptable view, if any view on 

 these insoluble questions can be regarded as acceptable, the 

 pectoral and pelvic fins are to be regarded as local specialisations 

 of a once continuous lateral fold of the body wall, containing 

 skeletal structures comparable to those of the unpaired fins, viz. 

 basal segmented somactids (radialia) and peripheral dermo- 

 trichia. This view was first suggested by Balfour. According 

 to it the fin-skeleton of Cladoselache would be appealed to with 

 its parallel somactids as being an obvious local specialisation of 

 a once continuous fold, with parallel somactids all along its 

 course. 



It would be useless to study the skeleton except in detail, 



* There appears to be some dispute as to whether this side is pre- or post- 

 axial. According to Wiodersheim and Fritsch, the side on which the 

 majority of the rays are placed is post-axial, but according to the more 

 generally received opinion it is pre-axial, the limb in the specimen from 

 which Fig. 87 is taken having been displaced. 



