THE SHARKS AND RAYS 91 



there is hardly sufficient evidence to establish its presence on 

 the occasion. All that can positively be said is that sword- 

 fishes have occasionally been found prowling in the neighbour- 

 hood of dying whales, and the bodies of the great mammals 

 have also been found pierced and lacerated by their snouts. 

 This does not, however, prove any co-operation between the 

 sword-fish and shark. 



Little seems to be known of the thresher's breeding, but 

 it probably brings forth its young alive, like most sharks in 

 our seas. 



The Basking Shark {Selache maxima') is the largest and 

 least aggressive of sharks. In addition to the characters 

 already established for the family, the basking shark is dis- 

 tinguished by its immense size, small examples not being met 

 with in our seas, as well as by its large gill-openings and 

 curiously protruding snout, which is dented with small pores. 

 It is further characterised by large gill-rakers, with which we 

 shall find affinities in the herrings, and which play an important 

 part in its feeding. In colour it varies between brown, dark 

 blue, and grey, being lighter on the sides and lower surface, 

 and with sometimes some dull red on the snout. The tail is 

 keeled, and has the upper (not lower, as stated by Day) lobe 

 the longer and notched. There is also a small pit in the line 

 of the back, just in front of the tail. Its teeth lie in seven or 

 more rows embedded in the mucous membrane. 



The size and harmlessness of the basking shark when 

 unmolested suggests obvious analogies with the whale and 

 elephant. With the whalebone whales, indeed, the basking 

 shark has somewhat closer affinities, for its bone-like gill- 

 rakers act as a sieve and strain off the water from the plankton, 

 or floating food, precisely like the baleen strainers of the whale. 

 They may, in fact, be compared with the teeth in the gill-arches 

 of bony fishes, for, like them, they prevent foreign substances 

 from clogging the gills. The comb-like structure is practically 

 a grating of narrow plates. Like the majority of sharks, 



