CHAPTER LII: THE TOP SHELLS AND DOLPHIN 



SHELLS 



Family Trochid/E 



Shell top-shaped or conical, spiral, pearly within; oper- 

 culum thin, horny, spiral, usually circular, with a central nucleus; 

 head with short, broad snout, long tentacles and simple eyes; 

 radula well developed; three to five fleshy cirrhi on each side of 

 mantle margin; one gill, the left; eggs laid on rocks or glued to 

 seaweed in masses. 



A large family including many genera of littoral and deep- 

 sea forms. They are chiefly vegetable feeders, living on algae. 

 Chiefly tropical; the largest and handsomest species are distri- 

 buted in the Pacific and Indian oceans. Some are highly coloured 

 in handsome patterns; all are pearly inside. A little shell, 

 pearly and opalescent, from the East Indies, is gathered in quanti- 

 ties to be strung into necklaces. Large specimens of top shells 

 are favourite ornaments for cabinets; the outer shell substance 

 is usually ground off to reveal the pearl foundation of the closely 

 wound coils. Some species are used in the manufacture of brace- 

 lets, buttons and pearl ornaments. 



Genus TROCHUS, Linn. 



Shell solid, regularly conical, with high spire of many close 

 whorls, which are usually angled and decorated with beading 

 and bright colqurs in striking patterns; base of shell broad, 

 flattened; aperture oblique, angled, with spiral operculum; 

 columella twisted, its edges generally toothed or folded. 



The genus is mostly confined to the Old World tropics. 



The Great Top Shell (7. Niloticus, Linn.), the heaviest 

 and largest of the top shells, is very striking in markings and 

 coloration. The spire is sharp, the whorls white under zigzag 

 radiating bands of red, violet or brown. The outer whorl flares 

 decidedly, and has more and narrower stripes than the upper 



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