PELAGIC FISHES. 297 



the Indo-Pacific basin is very much greater than in the Atlantic. 

 The extensive development of reef-structures through the former 

 area, presenting unusually favourable conditions for existence, has, 

 doubtless, much to do with this comparative superabundance. 



Pelagic Fishes. Our knowledge respecting the fishes which 

 spend a considerable, or the greater, part of their existence on the 

 free surface of the oceans, borne resistlessly in the course of the 

 oceanic current, or inhabiting masses of floating sea-weed, although 

 still very meagre, is sufficient to indicate that the number of such 

 types is very limited. As with the other groups of fishes, they are 

 most numerous in the regions of high temperature, diminishing 

 rapidly as we proceed either north or south from the Equator. 

 Most of the tropical genera are also met with in the temperate 

 zones, and probably the converse is also true, although a number 

 oi exceptions have been indicated. The warm-water fishes become 

 rare beyond the fortieth parallel, and decline very rapidly with the 

 decline of the temperate fauna itself. Almost the only pelagic fish 

 of the Arctic Ocean is the Greenland shark (Laemargus borealis). 



Among the better known bony-fishes that enter into the com- 

 position of the pelagic fauna are the flying-gurnards (Dactylopterus), 

 mackerels, tunny, bonitos, dolphins (Coryphsena), skip-jacks, pilot- 

 fishes, sword-fishes, frog-fishes, scopelids, skippers, flying-herrings 

 (Exoccetus), sea-horses, porcupine-fishes, and sun-fishes (Orthago- 

 riscus). The cartilaginous fishes are preeminently pelagic in their 

 habits, and contribute largely to this fauna. The sharks are repre- 

 sented by a number of genera (Carcharias, Galeocerdo, Zygsena, 

 Lamna, Notidanus, &c.), and by forms which are not only the 

 largest of their tribe, but approximately the largest of known fishes. 

 The basking-shark (Selache), the largest shark of the North At- 

 lantic, attains a length of more than thirty feet ; Carcharodon 

 Rondeletii, a tropical or sub-tropical species, the most formidable 

 of all sharks, has been known to measure forty feet, and Rhinodon 

 typicus, a species of the Indian Ocean, fifty to sixty feet. Rivals 

 to these monsters of the deep are the sea-devils or eagle-rays (My- 

 liobatidae), many of whose forms (Dicerobatis, Ceratoptera) appear 

 to attain a length of twenty feet or more. 



The similarity existing between the pelagic faunas of the At- 

 lantic and Indo-Pacific basins is very great, extending not only to 

 genera, but to species. A number of the forms are nocturnal in 

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