46 HANDBOOK OF SHELLS. 



from the spire and next the columella, is produced into a channel, 

 the anterior canal (as in Fig. 9) ; whilst where the mouth 

 meets the base of the spire there may be a kind of notch which 

 is termed the posterior canal. Most Gasteropods are dextral, 

 that is to say, the mouth is to the right of the axis as you look at 

 it ; a few, however, are sinistral, or wound to the left (like Physa); 

 whilst reversed varieties of both kinds are met with. 



Gasteropods of the first order have comb-like gills placed in 

 advance of the heart, and are hence termed PROSOBRANCHIATA. 

 They are divided into two groups : (a) Siphonostomata (Tube- 

 mouthed), in which the animal has a long proboscis, and a tube, 

 or siphon, from the breathing-chamber that passes along the 

 anterior canal of the shell, which in this group is well developed. 

 They have a horny operculum, or lid, with which to close the 

 aperture, (b) Holostomata (or Whole-mouthed). In these the 

 siphon is not so produced, and does not want to be protected ; 

 accordingly the mouth of the shell is entire, i.e. has no canal. 



The operculum is horny or shelly. 



The former (group a) includes 



several families : 



1. Strombidce, comprising shells, 

 like the huge Strombtis, or 

 "Fountain-shell," which is so 

 often used to adorn the mantel- 

 piece or rockery, and from which 

 cameos are cut. 



2. The Muricida, of which the 

 Murex (an extraordinary form of 

 this is the "Venus' comb," Murex 

 tennispina, Fig. 9), the Mitre- 

 shells, and the Red-Whelks(/W7/.r) 

 are examples. 



3. The Biiccinid(Z) taking its 

 name from its type, the Common 

 Whelk (Buccinum nndalittn), 

 and including such other forms 

 as the Dog- Whelk (Nassa), the 

 Purpnra, the strange Magilus, 

 and the lovely Harp- Shells and 

 Olives (Fig. 10). 



4. The Cassidida, or "Helmet- 

 Shells." Cassis rufa, from West 



Fig. o. Murex ten-iispina. AP 1 ?*,, f 



of shell for cameo engraving ; 



