THE SKELETON IN FISHES. PAIRED FINS. 129 



cartilaginous endoskeletal radialia are replaced by horny exo- 

 skeletal fin-rays. 



It is impossible here to give a full discussion of the rival 

 views, but some of the points which support Gegenbaur's view 

 may be mentioned. The fact that migration of visceral arches 

 has to be assumed is no difficulty, as it is obvious that 

 migration in the opposite direction has taken place in many 

 Teleosteans such as the Cod, whose pelvic fins are attached 

 to the throat in front of the pectorals. If migration did take 

 place, the pelvic fins being older than the pectoral should be 

 the more modified, and this is the case. Again, if the pectoral 

 girdle is a modified branchial arch, it must at some period 

 have carried a gill, and in Protopterus it does bear a vestigial 

 gill. 



According to the view more prevalent at the present time, 

 the paired fins have been derived from two continuous folds 

 of skin and their skeletal supports running forward from the 

 anal region along the sides of the body, their character being 

 similar to the fold that gave rise to the median fins. In 

 support of this view it may be argued that the paired and 

 unpaired fins are often identical in structure, and that some 

 Elasmobranch embryos do show a ridge running between the 

 pectoral and pelvic fins. Then from this continuous fold two 

 pairs of smaller folds may have been specialised off, and in 

 each a number of cartilaginous radialia may have been de- 

 veloped. The fin of Cladoselache from the Carboniferous of 

 Ohio apparently illustrates this condition. It consists of 

 certain basal pieces which do not project beyond the body 

 wall and bear a number of unsegmented cartilaginous radialia, 

 which show crowding together and are sometimes bifurcated 

 distally ; they extend throughout the whole fin from the body 

 wall to the margin. From this fin the archipterygium might 

 be easily derived by the enlargement of one of the middle 

 radialia and the segmentation and partial fusion of them all. 

 Whether the archipterygium be a primitive or secondary 



R. 9 



