238 



FISHES 



ancestor, so that the frontal bone and the teeth were originally 

 of the same nature. 



There are also dermal fin-rays in addition to the rays be- 

 longing to the internal skeleton above mentioned. In the dog- 

 fish and its allies the dermal rays are extremely fine and 

 numerous fibres of horny character, situated in the skin on 

 either side of the fin, and the denticles extend over the surface 

 of the skin outside them. In the bony fishes the horny fibres 

 are not present, but there are dermal rays which are really 

 modified scales. In these fishes the internal skeletal rays are 

 much shortened and reduced in the median fins to mere nodules 

 while the fin is supported chiefly by the dermal rays. The 



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Fig. 20. — A, cycloid scale of Plaice, showing four zones of annual growth ; 

 B, ctenoid scale of Sole. 



latter are necessarily double as they are developed in the skin 

 of both sides of the fin, and at their bases they bestride the 

 ends of the internal skeletal radials. The basals persist as 

 bones which alternate with the spines of the vertebrae and are 

 therefore called interspinous bones, and in some Teleosts the re- 

 duced segments of the radials are united to the basals so that 

 the dermal rays articulate directly with the ends of the inter- 

 spinous bones. The caudal fin in Teleostei undergoes an in- 

 teresting modification from the heterocercal structure which, as 

 described above, is seen in sharks and dog-fishes. The dorsal 

 upturned end of the vertebral column is reduced and converted 

 into a single rod-like bone called the urostyle ; the ventral 



