214 TKE DOG-FISH 



D. The Skeleton of the Fins. 



1. The median fins. 



The skeleton of the dorsal fins consists of a series 

 of parallel rods of cartilage, the fin-rays, sloping up- 

 wards and backwards, and calcified along their axes. 

 The lower or basal ends of the rods often unite 

 together, and the dorsal ends support one or more 

 rows of closely fitting polygonal plates. On each 

 side of this median cartilaginous skeleton is a series 

 of slender elongated horny fibres, derived from the 

 skin. 



The other median fins are similar to the dorsal 

 ones, except that the cartilaginous elements may be 

 reduced to a longitudinal basal rod, or may even be 

 completely absent. 



2. The paired fins are essentially similar to the median fins, 



and are probably to be regarded as local enlargements 

 of lateral fins, which originally extended the whole 

 length of the body. 



It is convenient to take the posterior or pelvic 

 fins before the pectoral, as their structure is simpler. 



a. The pelvic girdle is a nearly straight bar of cartilage, 



placed transversely across the ventral surface of 

 the body, about half an inch in front of the cloaca. 

 The ends of the bar are produced forwards into 

 blunt processes, and bear on their posterior 

 surfaces the articular facets for the pelvic fins. 



b. The pelvic fins. The cartilaginous skeleton of each 



fin consists of a slightly curved basal rod, the 

 basi-pterygium, which lies along the inner side of 

 the fin, and articulates in front with the pelvic 

 girdle. Prom the outer side of the basi-pterygium 

 a series of parallel cartilaginous fin-rays, about 

 fourteen in number, run outwards and backwards, 

 supporting at their ends smaller polygonal plates. 

 The anterior one or two of the fin-rays may 

 articulate independently with the pelvic girdle. 



