GASTROPODA. 17 



The Melaniiclce. are freshwater Snails which abound in most tropi- Cases 

 cal and subtropical countries ; about 1000 species are known. They 38-41. 

 are mostly of dark colours, and are fond of muddy places. 



The Cerithiidce are chiefly marine forms, some, however, entering Cases 

 brackish water. About five hundred fossil species have been described, ■*'^"'*^- 

 some of them gigantic in comparison with any now living, of which 

 more than two hundred are known. 



The 8cala scalaris was formerly considered a great rarity, as Case 44. 

 much as £40 having been given for a single speci- 

 men, which might now be purchased for as many ^^' 

 pence. 



The " Worm-shells " ( Vermetidce) are a very ^k Cases 



peculiar family. Their shells can scarcely be dis- '^" 



tinguished from the shelly tubes which are formed 

 by certain species of marine worms, Ser^nda, &c. 

 They are free and spiral in early life, but after- 

 wards become distorted and generally attached to 

 rocks, stones, &c. A foot for walking purposes 

 therefore would be of no use ; consequently it 

 is more or less obsolete, serving only as a support Scala scalaris. 

 to the operculum. Case 44. 



The " Screw-shells " {Turritellidce) have elongate tapering shells ; Caso 48. 

 about 100 recent and 200 fossil species are known. One species only 

 {Turritella communis) is now found living on the British coasts. 



The Xenophoridce have the singular habit of cementing to the Case 49. 

 exterior of their shell, stones, pieces of coral, and fragments of other 

 shells ; hence they have been called " Carrier-shells," and, according 

 to the kind of material chosen, have been named " Conchologists " 

 and " Mineralogists." Beyond acting as a disguise, and consequently 

 as a protection, there does not appear to be any special utility in 

 thus adding to the weight of their own shells. The animals do not 

 glide like most other molluscs, but scramble along like the Strombs, 

 the form of their foot being small, divided into a front, expanded, 

 and a hind, tapering portion admirably adapted to the nature of the 

 ground on which they live, which usually consists of broken and dead 

 shells. 



The " Wing-shells " {StromUdce) are the largest of the Gastro- Cases 

 pods with a proboscis or non-retractile snout. They do not crawl 49-52. 

 like most other Gastropods, but progress by a sort of jerking move- 

 ment. They act as scavengers, feeding on decomposing animal matter. 



The Strombus gigas, or " Fountain-shell," occurs in great numbers 



c 



