X. — MOLLUSCA. 81 



for burrowing in sand when in quest of bivalves. There 

 are some fine specimens of Scalaria pretiosa, which was 

 formerly considered a great rarity, as much as £50 havino- 

 been given for a single specimen. The Violet Snails 

 are found floating about in every ocean. They feed upon 

 jelly fish and construct a gelatinous raft filled with air 

 bubbles. A number of varieties are found on the coast of 

 New South Wales. 



The Screw-Shells (TibrritellidcB) have elongate tapering 

 shells, and are found mostly in tropical climates ; a few 

 are found on the Australian coast. 



The Periwinkles ( Littorinidce) are found on every shore ; 

 they feed on all kinds of marine vegetation, and numerous 

 species are found in Australia. 



The Ceriiliiidce are chiefly marine forms, some however enter- 

 ing brackish water. Some of the species emit a bright 

 green fluid Avhen molested. 



The Melaniidce are fresh water snails ; they abound in most 

 tropical and sub-tropical countries. A number of the 

 species are found in Australia. 



The River Snails (Paludinidce) might be termed fresh- 

 water Periwinkles, as the animals of both are very similar. 

 They are ratlier shiggish, and are found at bottoms and 

 margins of ponds and rivers, feeding on decaying animal 

 and vegetable matter. 



The family of Calyptrceidce includes the Slipper Limpets 

 and the Cup-and-Saucer Limpets (CrucihulumJ. 



The Worm Shells ( Vermetidce) are a very peculiar family. 

 Their shells can scarcely be distinguished from the shelly 

 tubes, which are formed by certain species of marine 

 worms, Serpula, &c. They are free and spiral in early 

 life, but afterwards become distorted and generally attached 

 to rocks, stones, &c. 



