212 THE DOG-FISH 



The anterior one or two of the fin-rays may 

 articulate independently with the pelvic girdle. 



In the adult male, the posterior end of the 

 basi-pterygium is produced backwards into a large 

 spout-like process, supporting the clasper. 



Besides the cartilaginous skeleton, horny fibres 

 like those of the median fins are present in great 

 numbers on both surfaces of the fin. 



c. The pectoral girdle, which lies immediately behind 



the last gill-arch, is considerably larger than the 

 pelvic, and consists of a hoop of cartilage incom- 

 plete above. 



The ventral portion of the hoop is produced for- 

 wards into a flattened process, which forms part of 

 the floor of the pericardial cavity, and is hollowed 

 dorsally to receive the ventricle of the heart. 



At each side the arch is thickened, and bears 

 on its outer border a triple articular facet for the 

 basal cartilages of the fin. 



d. The pectoral fin has three basal cartilages, the pro- 



pterygium, meso-pterygium, and meta-pterygium, 

 articulating with the three facets on the pectoral 

 girdle. Of these the pro-pterygium is the smallest, 

 and the meta-pterygium, which extends along the 

 inner or attached border of the fin, is much the 

 largest. 



From the outer borders of the basal cartilages 

 a series of close-set cartilaginous fin-rays diverge : 

 of these the pro-pterygium supports a single large 

 one ; the meso-pterygium also supports one, which 

 is, however, split distally into two or three; while 

 the meta-pterygium supports twelve or more much 

 narrower rays. 



To the outer ends of the fin-rays two or more 

 rows of polygonal plates of cartilage are attached, 

 forming the peripheral portion of the skeleton. 



Horny fibres are present in large numbers, as 

 in the pelvic fins. 



