i 7 8 



EVOLUTION OF ELASMOBRANCHS 



VI. 3 



pectoral girdle. The tail was heterocercal and there are no signs of skele- 

 tal support for lateral keels. All of these Harris regards as primitive 

 features; *Diademodus was specialized in having no spines in front 

 of the dorsal fin and no clasper on the head. Both of these features are 

 frequent in hybodonts and in *Cladoselache there is a large spine 



zd nsi 



Fig. 112. Development of the fins of the dogfish, i, Adult showing the nerve-supply 

 of the fins ; 2, adult with the fins shown expanded and their nerves and muscles shown 

 as if concentration had not taken place; 3, a 19-mm. embryo, showing the actual 



condition. 



a. anal fin; ac. anterior collector nerve of first dorsal fin; cr. (black) cartilaginous radial 



partially hidden by the radial muscle; n. 1-57, spinal nerves and ganglia; pc. collector 



nerve of second dorsal fin; pi. pelvic fin; pt. pectoral fin; rm. radial muscle; id. and 2d. 



first and second dorsal fins. (From Goodrich, Vertebrata, A. & C. Black, Ltd.) 



in front of the first dorsal fin. These fishes were thus like modern 

 sharks in their general form, but the fins were remarkable in having 

 a broad base, not sharply marked off from the body- wall. It has been 

 suggested by Goodrich and others that this was the earliest condition 

 of the pectoral fin, perhaps showing its derivation from a continuous 

 or extended fin-fold (Fig. 113). This theory has the advantage that it 

 agrees with the embryological development of the fin by concentration 

 of a series of segments (Fig. 112). It also seems likely that anterior 

 and posterior fins expanded in the horizontal plane would be neces- 

 sary for stabilization (p. 136). Moreover, this theory of the origin of 

 paired fins has the great advantage that it compares them with the 

 median fins, which are also continuous folds. It has been argued, 



