HANDBOOK OF SHELLS. 



Fig. 6. Loligo viilgaris, and " Pen." Fig. 7. Sepia officinalis. 



whose so-called " bone " (once largely used as an ink eraser) is 

 frequently found on our southern coasts ; and the pretty little 

 Spirula (Fig. 8). 



The only representative of the four-gilled order now living 

 is the well-known Pearly Nautilus ; but in former times the 

 Tetrabrar.chiata were extremely numerous, especially the Am- 

 monites. 



CLASS II. GASTEROPODA (Belly-footed) comprises those 

 mollusca which, like the common snail, creep 

 on the under-surface of the body, and with one 

 exception (Chiton, Fig. 20) their shells are uni- 

 valve (i.e., composed of one piece). But before 

 we go further, it may be well to point out the 

 names given to different parts of a univalve shell. 

 The aperture whence the animal issues is called 

 the mouth, and its outer edge the lip ; each turn 

 of the shell is a whorl ; the last and biggest, the 

 body-whorl , the whorls, from the point at the Fig. 8. Spirnla. 

 top, or apex, down to the mouth form the spire ; and the line 

 where the whorls join each other is called the suture. The axis 

 of the shell around which the whorls are coiled is sometimes 

 open or hollow, and the shell is then said to be umbilicaUd (as 

 in Fig. 3^) ; when closely coiled, a pillar of shell, or columella, is 

 leU (as in Fig. 9). Sometimes the corner of the mouth farthest 



