FINS OF GA.KOIDS AND TBLEOSTS. 585 



In the Arthrodira (e. g. Coccosteus) [Smith Woodward, 12] the 

 radial elements are very similar bisegmental structures. 



In existing Elasmobranchs the typically rod-like cartilaginous 

 radial elements are generally trisegmental, exhibiting a division 

 into proximal, mesial, and distal segments, flexibly connected with 

 one another by ligament, and in fairly close apposition throughout 

 their length for mutual support. The central or approximately 

 central elements are usually the longest, and almost invariably 

 the most anterior and posterior undergo reduction in length and 

 lose one or more of their constituent segments — facts wliich find 

 their legitimate explanation in the partial atrophy of an origi- 

 nally more extensive fin and the concentration of the persistent 

 residue of the fin-supports. The horny fibres, as was probably 

 also the case in the fossil Elasmobranchs above-mentioned, are 

 much more numerous than the supporting cartilages, and to 

 a greater extent than in extinct types they supplant the latter in 

 supporting the flexible peripheral margins of the fins. As has 

 already been pointed out, the radial elements are liable to con- 

 siderable modifications in different genera through {a) the longi- 

 tudinal concrescence of the proximal segments, or of both proximal 

 and mesial ; (h) the suppression of particular segments in certain 

 of the elements ; and (c) the apparently secondary subdivision of 

 the distal segments. 



The polymorphic character of existing Ganoids is well illus- 

 trated by the existence of striking variations in the structure of 

 the radiaj elements, of which three well-marked types are repre- 

 sented within the limits of the group. 



(1) In Acipenser and Polyodon the trisegmental radial ele- 

 ments are essentially similar to tliose of Elasmobranchs in shape 

 and mutual relations, in the large relative size of the mesial 

 segments, the tendency to occasional concrescence on the part of 

 the proximal segments, tlie excess in the number of dermal fin- 

 rays which they support, and also in the fact that the cleft bases 

 of the fin-rays embrace between them not only the distal but to 

 some extent the mesial segments also. On the other hand, there 

 are not wanting indications of increasing specialization in the 

 partial ossification of the proximal and mesial segments, and the 

 reduction of the distal segments to the condition of simple carti- 

 laginous nodules. The fin-rays also exhibit modifications in 

 the same direction. Not only are they partially ossified, but, 

 although more numerous than the supporting radial elements, 



