io8 



PISCES 



And parallel (orthostichous) somactids or radials of the paired-fin 

 folds, becoming concentrated, fused at their base, giving rise to a 

 ' pleurorachic ' and ' monostichous ' type of skeleton (Fig. 68). 

 In such a fin, the axis lay in the body-wall, and bore a single row 

 of radials along its outer edge. This type of fin skeleton may be 

 found in the median fins of Elasmobranchs and other fish (p. 75 

 and Figs. 87, 1 50), but not in the pectoral fin of any living form. The 

 pelvic fins, however, in Elasmobranchs and the lower Actinopterygii 

 very nearly conform to it (Fig. 96). But among the early fossil 



FIG. 68. 



Diagrams showing the possible derivation from each other of the various types of pectoral 

 fin skeleton in the Osteichthyes. A, pleurorachic type (Cladodiis). B, hypothetical stage leading 

 to the mesorachic type C (Ceratodits). D, hypothetical type leading to E (Acipenser, Amid). F, 

 teleostean type, reached either from A through E, or from C through D and E. a, segment of 

 axis ; b, basal of axis ; g, pectoral girdle ; pr.r, preaxial radial ; pt.r, postaxial radial. 



Chondrichthyes (Cladoselachii, Fig. 155) pectoral fins are found 

 which possibly have a skeleton of this character (Dean [104]). 

 Philogenetically, the pleurorachic fin could become mesorachic by 

 the freeing of its hinder edge from the body-Avail, and by the 

 shifting of the axis towards the centre of the fin-lobe, so that 

 radials should come to develop on the growing postaxial side. 1 

 The rhipidostichous fins would be, to some extent, intermediate 

 forms (Fig. 68). The chief objection to this view, and it is a 



1 The ontogenetic formation of the axis in one region of the fin or another is, as 

 Mollier has shown [301], due to the concentration of the radials about a different ideal 

 axis. In the Rajidae there may be two axes in one fin (p. 128 and Fig. 121 ). 



