16 THE ELASMOBRANCH FISHES 



of the anterior radials. Similarly Balfour has demonstrated for Scyllium that 

 the pectoral girdle arises secondarily. It may also be added that the blood sys- 

 tem of the paired fins in a generalized type like Heptanchus and Hexanchus 

 can most readily be interpreted in terms of a lateral fin-fold theory (see Bib- 

 liography p. 195, Daniel, 1926). Other characters of musculature and nerve 

 may be siid to bear on this subject. To them we shall direct further attention 

 when we consider these systems. 



