MOLLUSCA. 25 



If we design tlie Aquarium to be of any service to 

 lis in tlie observation of its contents, this growth must 

 be got rid of, or we might as well have a vessel with, 

 opaque sides. Here then comes in the aid of the 

 Periwink. Exclusively a vegetable-eater, he delights 

 in the green sea-weed, and nothing can be more con- 

 genial to his palate than these tender succulent 

 growths. The little Yellow Winkle that I first spoke 

 of possesses a similar appetite ; but he is less suitable 

 for the service required, inasmuch as his constitution 

 appears unable to bear constant submersion ; his habit 

 is to live a good deal exposed to the air, and even to 

 the hot sun, and this seems essential to his health. I 

 have found that if this little species be collected, pretty 

 as the individuals are, they crawl around the sides for 

 a day or two, as if seeking a more genial dwelling, and 

 then one by one fall to the bottom and die. There 

 is, however, another genus of Univalve Mollusca which 

 may be made equally available with the Periwink, if 

 indeed it be not superior for the purpose; I allude 

 to those evenly conical shells, which belong to the 

 genus Trochiis, sometimes called, from their form, 

 Tops. Two species, T. cinerarius and T. umhilicatus, 

 are scarcely less abundant on om* weedy shores than 

 the Periwinkles ; the former of a dull purplish grey, 

 marked with close-set zigzag lines ; the latter rather 

 flatter, usually worn at the summit, of a dull olive or 

 green, with narrow reddish bands radiating from the 

 centre. Both are pearly in tbe interior, but the latter 

 species is brilliantly iridescent. 



These Tops and the common Periwink are very 

 useful inhabitants of a marine tank \ they make them- 



