UNIVALVE MOLLUSCA. 433 
About fifty species are known, some of them large shells, others very 
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Fig. 221.—Trochus Cookii (Chemnitz). 
Fig. 222.—Trochus imbricatus (Gmel.) Fig. 223.—Phorus conchyliophorus (Borfu). 
small. Zurbo margaritaceus (Fig. 224) is a large, thick, and weighty 
shell, round-bellied, and deeply furrowed ; in colour it is yellow, or 
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