32 



MOLLUSCA. 



If we design the Aquarium to be of any service 

 to us in the observation of its contents, this growth 

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

 vessel with opaque sides. Here then we bring in 

 the aid of the Periwinkle, which may be bought 

 alive of any London fishmonger, half-a-pint for a 

 penny. Exclusively a vegetable-eater, he delights 

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

 congenial to his palate than these tender succulent 

 growths. The little Yellow Winkle, so abundant 

 on weedy rocks, 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 ex- 

 posed 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 indivi- 

 duals 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 MoUusca 

 which may be made equally available with the 

 Periwinkle, if indeed it be not superior for the 

 purpose. I allude to those evenly conical shells, 

 which belong to the genus Trochus^ sometimes 

 called from their form, Tops. Two species, 71 

 cinerarius and T. umhilicatus^ are scarcely less 

 abundant on our weedy shores than the Peri- 

 winkles ; 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 the interior, 

 but the latter species is brilliantly irridescent. 



These Tops and the common Periwinkle are very 



