UNIVALVE MOLLUSCA. 453 



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



Fig. 218. — Trochus niloticus (Linnaeus). 



Fig. 220.— Trochus inermis (Gmel.) 



Fig. 219. — Trochus virgatus (Gmel. 



Fig. 221. — Trochus Cookii (Chemnitz). 



Fig. 222. — Trochus imbricatus (Gmel.) 



Fig. 223. — Phorus conchyliophorus (Borfu). 



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

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



c c 



