96 GASTROPODA. 



wide ocean, and is a mere visitor to us. But 

 from the sea-bottom the dredge gathers many 

 species. On a sandy ground the tusk-like Denta- 

 Hum, the polished Natica, the Wentletraps, and 

 the different sorts of Bulla are found, as most of 

 these have burrowing habits. If the bottom be 

 rough and stony, the great Whelks and Spindles, 

 the Pelican' s-foot (AporrJiais) , the Turritella, and 

 many others, come up in the dredge. But the 

 richest ground of all is an old Oyster or Scallop 

 bed, where we may obtain the curious Cup-and- 

 saucer (Calyptr&a), the Keyhole Limpet (Fis- 

 surella), the Notched Limpet (Emarginula) , the 

 Torbay Bonnet (Pileopsis) , and many others of 

 interest, adhering to the old shells, among the 

 zoophytes, stars, and worms, which throng such 

 prolific localities. 



Identification. The most prominent points for 

 discriminating the genera are these. The presence 

 or absence of a shell ; its form ; the direction of 

 its spire ; the presence of an operculum ; its sub- 

 stance ; its mode of formation ; the presence or 

 absence of a proboscis ; the extent of the mantle ; 

 the number and shape of the tentacles ; the form 

 and position of the gills ; the extent and form of 

 the foot ; the nature of the shell-axis ; the position 

 of the eyes ; and finally the tongue, its compara- 

 tive length and breadth, and the form and ar- 

 rangement of its teeth. 



Authorities. These are the same as those cited 

 for the CONCHIFERA. 



GASTKOPODA. 



Molluscous animals furnished with a distinct 

 head, and a fleshy crawling foot ; either naked or 



