10 GIANT FISHES 



MACKEREL SHARKS. 



(Family Lamnid/E.) 



Very similar to the Sand Sharks, but with a stout, torpedo- 

 shaped body, and usually with a strong keel on each side of the 

 tail. The spiracles are minute or absent altogether. Where 

 the external gill-clefts are small, the fifth or last is placed just 

 above the root of the pectoral fin ; where they are large, the 

 last extends downwards just in front of the base of the fin. 

 The second dorsal fin is much smaller than the first, and is 

 placed nearly opposite to the anal. The caudal fin is roughly 

 moon-shaped, with the lower lobe but little shorter than the 

 upper, or the upper lobe is very long and the lower quite 

 short. The pectoral fins are sickle-shaped. 



These are large surface-swimming sharks, found in most of 

 the seas of the world. All are believed to be viviparous. Most 

 of the genera appear to date back to the Cretaceous period. 



The general shape of these sharks, with their powerful tails, 

 suggests active, predaceous creatures, capable of producing a 

 high rate of swimming when required. The torpedo-like, 

 streamlined body is eminently suited for rapid progression in a 

 comparatively dense medium such as water, and the pointed 

 snout overhanging the mouth provides an efficient cutwater 

 which greatly facilitates progress. The coloration is practically 

 the same in all the members of the family, being a dark bluish- 

 grey on the back, shading away gradually to white on the lower 

 parts. This is the typical coloration of most pelagic fishes, 

 that is to say of fishes that habitually live at or near the surface 

 of the sea, and illustrates the principle of what is known as 

 obliterative shading. The shading is exactly the reverse of 

 that which is produced by light thrown upon the fish from 

 above, and its general effect is to destroy the appearance of 

 thickness and to 'make the creature appear as a flat object. 

 This type of coloration also tends to conceal the fish in its 

 natural surroundings, for, seen against a background of dark 

 water coloured very much like its own back, it is almost 

 indistinguishable. 



