vi. 5 



SHARKS 



181 



but without placentae. Chlamydoselache, the frilled shark, lives in 

 deep water and feeds on cephalopods. The division Galeoidea is much 

 larger and includes the sharks with two dorsal fins, not supported by 

 spines. Here belong the dogfishes Scyliorhinus and Miistelus, both 

 mainly bottom-living animals feeding on a mixed diet, including 



Fig. 114. Teeth of various elasmobranch fishes. 



1, Man-eater (Carcharodon); 2, tiger shark (GalaeocerJo); 3, comb-toothed shark (Hexanchus); 



4, sand-shark (Odontaspis); 5, blue shark (Carcharinus); 6, nurse shark (Ginglymostoma); 



7, guitar rish (Rhina), 8, eagle-ray {Myliobatis), (After Norman.) 



crustaceans and molluscs. In Cetorhimis, the basking shark, the pre- 

 daceous habit of the group has been abandoned in favour of straining 

 small food directly from the plankton by means of special combs on 

 the gills (gill rakers), an arrangement recalling that of the whalebone 

 whales. The great effectiveness of this method of feeding may be seen 

 in the length of 35 ft or more attained by some of these sharks. Bask- 

 ing sharks produce very numerous small eggs, which develop within 

 an 'uterus', but without placentae. Rhineodon, the whale shark, is also 



