TYPICAL GROUP. 523 



expanded; and the spiracle is always small and may be wanting. The front 



teeth, and very frequently also those on the sides of the jaws, are formed on the 



type of a laterally compressed cone with cutting edges, at the base of which two 



or more smaller cones maj^ be developed ; but in one existing and many extinct 



genera the hinder teeth have blunt crowns adapted for crushing. The two branches 



of the jaws do not run parallel to each other, in consequence of which the teeth 



form oblique rows, whereas in the rays they are set in straight longitudinal rows. 



From the other sharks the members of the present family may be distinguished 



by the absence of spines in both the dorsal fins, of which the first is situated above 



the interval between the pectoral and pelvic pairs ; by the presence of a nictitating 



membrane to the eye ; and by the teeth, when fully formed, being hollow, and 



usually pointed. The bony elements in the skin take the form of minute granules, 



thus constituting the well-known " shagreen," as the dried skin is termed. 



In all the members of the tvpical j,'enus (Carchw'ias) 

 Typical Genus. . >/ sr o \ 



the muzzle is produced forwards, and the inferiorly-placed mouth 



is crescentic and armed with large, flat, triangular, single-coned teeth, of 

 which the upper ones differ considerably in form from those of the lower jaw. 

 Spiracles are absent, and there is a pit at the root of the caudal fin, which has a 

 distinct lower lobe. At the present day these sharks are represented by between 

 thirty and forty species, of which the blue shark is one of the commonest and 

 most widely distributed ; while in a fossil state the genus is known from the 

 Tertiary formations. The blue shark frequently attains a length of from 12 to 

 15 feet, but some of the other species arc stated to grow to as much as 25 feet. 

 In common with the other larger members of the suborder, all these sharks are 

 more abundant in tropical than in temperate seas ; but the blue shark is by no 

 means an uncommon visitor to British waters, more especially on the southern and 

 western coasts of Ireland. Mr. J. T. Carrington writes that they more usually 

 wander to the British coasts "in warm weather, especially in autumn, but they 

 have been seen in June, and even in the month of March. They are nocturnal in 

 their more active habits, taking rest and sleep in the daytime, often on the surface 

 of the water, with a portion of the dorsal fin and extremity of the tail exposed 

 above in the air. So gentle are they in their movements that, unlike many other 

 monsters of the deep, they do not disturb the luminous creatures, which at the 

 same time will be lighting every wavelet with their phosphorescence. Blue 

 sharks are not very particular as to what fish they take as food, though those 

 which are gregarious in their habits, like mackerel, pilchards, and herring, are 

 most commonly hunted by them. It is on record that big fish, such as congers 

 and the larger dog-fish, were found in a dead specimen from Cornwall. Occasion- 

 ally they become entangled in the drift-nets set by the pilchard-fishers, but these 

 sharks will also take a bait. Great care is necessary in landing a hooked specimen, 

 in case it gives a blow witli its tail, which may result in serious consequences, such 

 as broken limbs or ribs." It is a somewhat remarkable fact that in places like 

 Aden, where sharks of various kinds abound, the natives will swim ami dive 

 fearlessly in the open sea. where a European would be almost instantly devoured 

 by these monsters. The blue shark has the whole of the upper-parts slaty blue, 

 and the under surface white. 



