106 THE STORY OF ANIMAL LIFE 



lish molluscs it is comparatively soft and semi- 

 transparent, and reminds one of a thin slice of 

 horn. In many tropical molluscs, however, it is 

 hard and shelly. The large tropical shells named 

 Turbo have massive lids of considerable weight. 

 These shells, which are nearly allied to the pearly 

 Top-shells (Trochus) of the English shores, are 

 sold as ornaments, the outer coat of the shell 

 being partly scraped off to show the inner coat of 

 pearl : it is rarely, however, that the purchaser 

 obtains a lid, or even knows that the creature had 

 one. The reverse is the case with some of the 

 smaller kinds, the lids of which, being brightly 

 coloured, are imported without the shell, and 

 sometimes set as articles of jewellery. Some of 

 these are of a bright green hue. 



While the lids of the Holostomata are rounded 

 in shape, those that belong to the Siphonostom- 

 atous shells are necessarily more or less modified 

 so as to fit the mouth of the shell, and are conse- 

 quently oval or even claw-like in shape. The 

 Sting-winkle already spoken of, the common small 

 whelk, Purpura lapillus, and the large whelk, J3uc- 

 cinum undatum, are common shell-fish in which the 

 elongated lid may be studied. The lid is not, 

 however, like the tongue-ribbon, an essential fea- 

 ture of the structure of every univalve mollusc.* 

 Not only are there special instances in which it is 

 greatly modified, but also there are whole groups 

 of univalve molluscs in which it is absent. 



A curious suggestion has been made with 

 regard to the lids of univalve shell-fish ; namely, 



* There are one or two exceptional cases of gasteropod 

 molluscs that have no tongue-ribbon. The majority of these 

 are parasitic forms, which can get their food without the trouble 

 of filing it down. 



