124 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



radials on the anterior side of the fin. The ' archipterygial theory ' of Gegen- 

 baur assumes a biserial appendage (fig. 134) like that of Ceralodus (the *archi- 

 pterygium') as the type from which all legs and other fins have been derived, by a 

 shortening of the axis and a loss of radials, chiefly on the preaxial side. The 

 two views are illustrated in the adjacent sketches (fig. 133). No known facts 

 of either embryology or paleontology throw any certain light on the matter. 



Cladoselache (fig. 118) and the lower ganoids 

 have what is apparently the most primitive type 

 of fin with a large number of basalia which sup- 

 port a large number of radialia. From these, 

 as we go upward in the scale, there is a reduc- 

 tion in the number of basalia, either by disap- 

 pearance or fusion, while the other parts are 

 variously modified. Thus in recent elasmo- 

 branchs the characteristic number of basalia is 

 three in the pectoral, two in the pelvic fin. 

 These are known, from in front backward as the 

 pro- meso-, and metapterygium, the middle one 

 being absent from the hind limb. The numerous 

 radials are jointed transversely (fig. 120), per- 

 mitting more flexibility, and these may be arranged 

 entirely on one side of the basalia (uniserial) 



^i 



Fig. 134. Fig. 135. 



Fig. 134. — Pelvic fin and part of girdle of Ceratodus, after Davidoff. a, axial skele- 

 ton of fin; pil, iliac process; pint, processus impar; r, radialia. 



Fig. 135. — Skeleton of pectoral fin of Xenacanthus, after Fritsch. 



or the metapterygium may be prolonged as an axis, and while 

 most of the radialia are on the preaxial side, some may occur on 

 the postaxial side (biserial) as seen in the carboniferous shark, 

 Xenacanthus (fig. 135). In the recent species the skeleton of the 

 fin is continued by actinotrichia. In the male elasmobranchs the 

 pelvic fin is divided into two lobes, the medial, the so-called clasper 

 (mixipterygiimi) being the longer and narrower. This is used in 



