414 H. B. POLLARD, 



Shoulder girdle and Pectoral fin (fig. 16—18). 



The true dorsal surface of the fin is directed backwards and is 

 unprovided with scales. The true ventral surface looks forwards and 

 is provided with light coloured scales, which must be considered con- 

 tinuous with the ventral scales of the body. These scales pass some- 

 what round the pro- and metapterygial edges of the fin. The fin is 

 twisted in the same direction as the fin in Teleostei but through a 

 greater angle. 



The muscles show signs of a differentiation from the primitive 

 Selachian condition but the diflerentiation does not proceed very far. 

 The following elements may be distinguished: 



1) A levator muscle taking its origin from the inner surface of the 

 dermal clavicle and proceeding to the upper or propterygial edge of 

 the fin belonging rather to the posterior or true dorsal surface. 



2) Protractor or extensor muscles over the anterior or true ventral 

 surface the superficial portions taking their origin from the edge of the 

 dermal clavicle and proceeding to the distal cartilaginous rays of the 

 fin, that is, to where the bony rays commence. Deeper portions arise 

 from the edge of the metapterygium and especially from the meso- 

 pterygium proceeding to the same region as the superficial muscles. 



3) A depressor muscle arising from the coracoid and lower part of 

 the dermal clavicle and affixing itself to the metapterygium. Some 

 tendons arise from the fasciae of the body muscles posteriorly. 



4) Retractor or flexor muscles corresponding on the posterior sur- 

 face to the extensors on the anterior. The nerve supply to the fin is 

 from the hypoglossal and first 4 spinal nerves. The Xllth or hypo- 

 glossal nerve as it passes down behind the gills gives off first a fine 

 commissure to the 1st spinal (primitively the 3rd) and then divides 

 into 3 branches one of which continues forward to join the Xlth nerve 

 and supply the geniohyoid. The second runs down above the pro- 

 pterygium and supplies the uppermost (anterior) portion of the pro- 

 tractor. The 3rd branch joins the first spinal nerve. The 1st spinal 

 nerve runs in a straight course to this anastomosis and then two 

 branches pass on one of which (possibly mostly derived from XII) supplies 

 the levator muscle and then proceeds to the anterior or ventral sur- 

 face supplying the protractors. The second branch runs straight over 

 the dorsal hinder surface and supplies the flexors or retractors. 



The 2nd and next spinal nerve runs straight to the fin and is some- 

 what deeper placed than the others. Near the fin it divides into 2 



