PLE URA CA NTHODII 



181 



(Fig. 146). W ell-developed arches 

 occur above and below, but the 

 intercalaries are small or absent ; 

 there are short ribs. The neural 

 spines articulate with a series of 

 dorsal radials formed of three seg- 

 ments. In the caudal fin these radials 

 are segmental ; but in the dorsal fin 

 they are twice as numerous as the 

 arches. No such radials have been 

 found in the ventral lobe of the 

 caudal, a fact which argues against 

 the fin being truly diphycercal. 



In the region of the anal fins the 

 radials, still articulating with the 

 haemal arches, become fused in a 

 variable manner, showing most 

 clearly how such concrescence may 

 lead to the formation of a fin-axis 

 (Fig. 147). 



Behind the occipital spine, which 

 seems to have been movable, the 

 neural arches point forwards. 



The two halves of the pectoral 

 girdle remain separate, and each has 

 a small dorsal and ventral segment 

 (Fig. 148). Most interesting is the 

 skeleton of the fin, which is built 

 on the archipterygial or mesorachic 

 plan, with a tapering jointed axis 

 bearing a preaxial series of radials 

 in front and a postaxial series behind. 

 The preaxial are the more numerous, 

 and some of them may be articulated 

 to the girdle itself. The pelvic girdle 

 is formed of two separate cartilages 

 (Figs. 149, 150, 151). The skeleton 

 of the pelvic fin never appears to 

 be of the mesorachic type; although 

 the radials have evidently been con- 

 centi-ated in much the same manner 

 as in the pectoral to form a seg- 

 mented axis, there is no series of 

 postaxial radials. One postaxial 

 radial has, however, been described 

 (Fig. 150). In the male the axis 



