VENUS' -CO MB SHELLS, WHELKS, ETC. 637 



The ATLANTID.E have convolute shells, which in Atlanta is flat, and 

 furnished with a single remarkably deep keel at the periphery, and in 

 Oxygyrus is globose, and provided with many small keels. 



The PTEROTRACH^ID.E, as a rule, have neither mantle, shell, nor tentacles; 

 in Carinaria (Fig. 16), however, there is a cap-shaped, transparent, glassy 

 shell covering the small visceral mass. The animals are carnivorous. 



To the Tsenioglossa, also, in all probability, belong the families CIIORISTID^E, 

 ADEORBID.E, SEGUENZIID/E, and OOCOIIYTHID^:. 



Sub-order 4. RACHIGLOSSA, Mollusca with long retrac- 

 tile proboscis. The formula for the radula is I'l'l; 

 the laterals, however, are often wanting. 



The MURICII^E are a numerous family, chiefly familiar 

 by the many examples of the spiny species of Murea 

 that adorn some people's houses. Their shells have 

 moderate spires and generally a large body-whorl ; the 

 mouth is furnished with both anterior and posterior 

 canals, the former often being of great length, and a 

 horny operculum. They are usually more or less 

 spirally ribbed, whilst Murex has numerous strong 

 varices frequently ornamented with spines of great 

 length, which, in the Venus* Comb (Fig. 17), extend the Fi(j 17 



whole length of the anterior canal. The dog-periwinkle, MDRBX TENUI-SPINA. 

 Purpura lapiUus, so common on our coasts, belongs to 

 this family. 



The CORALLIOPHILTDJE live amongst masses of coral ; Magilus, indeed, 

 becomes buried in them, and forms a tubular shell to keep pace with the 

 coral as it grows. The young Magilus, before it becomes fixed, and the 

 other members of the family resemble Purpura, both in shell and animal, but 

 have no radula. 



The COLUMBELLIDJE are small fusiform shells resembling Strombus in shape ; 

 but the lip, instead of being expanded, is thickened and dentate within ; the 

 aperture is narrow and the anterior canal short. 



The NASSIDJS, or dog-whelks, have a shell like the common whelk, only 

 smaller ; the outer lip is thickened and dentate ; the columella lip is 

 thickened with a callus ; the operculum is horny ; the wide, thin foot 

 of the animal is divided behind into two tails. Nassa reticidata is very 

 common on our coasts. 



To the BUCCINID.E belong the common whelk and its allies. The siphon 

 is rather long and allows the animal to burrow in the mud in search of the 

 bivalves on which it feeds, boring a neat little hole in their shells and rasp- 

 ing out its victim with the radula in its long, retractile proboscis. The foot 

 is large and thin ; the operculum horny and oval ; a scanty, rough 

 periostracum is found on fresh specimens, but soon rubs off. 



The TURBINELLID.E have generally pear-shaped, solid shells, usually with 

 shelly plaits or folds on the columella ; the anterior canal is fairly long. 

 They are for the most part tropical forms. To this family belongs the 

 largest known Gastropod, Semifusus proboscidiferus, which attains 2 ft. in 

 length. It comes from Australia. 



The FASCIOLARIIDJE have elongate fusiform shells ; the spire is generally 

 high and pointed and the anterior canal long. The columella bears several 

 shelly plaits or folds ; the lip is simple, and the horny operculum bluntly 

 claw- shaped. The whorls are frequently ornamented with tubercles arranged 



