584 PEOF. T. W. BEID&E ON THE MESIAL 



Each fin-ray is cleft longitudinally for the proximal three-fourths 

 of its length, and its lateral halves expand towards the base of 

 the ray into thin splint-like plates, and firmly embrace between 

 them for nearly its whole length one of the distal radial 

 segments. In striking contrast to their massive supporting 

 cartilages, the posterior two or three rays are very feebly 

 developed. 



Anal fin. — In the anal fin there are eleven proximal radial 

 segments, and fifteen distal segments supporting a like number 

 of fin-rays. Except for the partial fusion of the first two 

 proximal radial segments, the fin and its fin-supports differ but 

 little from the description of the dorsal fin given above. 



III. SUMMARY. 



In this section it is proposed to institute a comparison of the 

 principal modifications of the radial elements of the mesial fins 

 with regard to their degree of segmentation, the extent to which 

 they are affected by degeneration and concrescence, and the 

 variable modes of support they offer to the fin-rays, in different 

 groups of Eishes. 



The most primitive type of radial element is to be found in the 

 Marsipobranchs, where they exist in the form of unsegmented 

 cartilaginous rods, either simple or dichotomously branched 

 towards their distal ends, and, in the absence of horny fibres or 

 fin-rays, they extend to the peripheral margins of the fins and 

 constitute their sole skeletal support. 



In retaining the condition of simple unsegmented cartilaginous 

 rods, the radial elements of the Holocephala resemble those of the 

 Marsipobranchs ; but how far the simplicity of these structures 

 is primitive, or has been acquired by the suppression of segments, 

 cannot at present be determined. Actinotrichia in the form of 

 horny fibres support the periphery of the fins. 



In the most primitive of extinct Elasmobranchs (e. g. Glado- 

 selacJie, Fleuracanthus) the radial elements of the dorsal fins 

 become complicated by segmentation, each being divided into a 

 basal and a distal segment, of which tlie distal is the longer. As 

 pointed out by Dean [11], the various elements extend to the 

 periphery of the fin and in conjunction with horny fibres, which 

 in CladoselacTie are of secondary importance and lie between the 

 former, contribute to the support of the fin. 



