UNIVALVE MOLLUSC A. 433 



About fifty species are known, some of them large shells, others very 



Fig. 218. Trochus niloticus (Linnseus). 



Fig. 219. Trochus virgatus (GmeL 



Fig. 220. Trochus inermis (Gmcl.) Fig. 221. Trochus Cookii (Chemnitz). 



Fig. 222. Trochus imbricatus (Gmel.) Fig. 223. Phorus conchyliophorus (Boifu). 



small. Turbo margaritaceus (Fig. 224) is a large, thick, and weighty 

 shell, round-bellied, and deeply furrowed ; in colour it is yellow, or 



