HANDBOOK OF SHELLS. 



49 



'Tooth-shell," are simply slightly curved tubes, open at both 

 ends and tapering from the mouth downwards, and cannot be 

 mistaken. 



19. Lastly, we have the Chitonidts, whose single genus Chiton 

 possesses shells differing from all other mollusca in being com- 

 posed of eight plates overlapping each other, and in appearance 

 reminding one of the wood-louse. This animal is not only like 

 the limpet in form but also in habits, being found adhering to 

 the rocks and stones at low-water. 



Order II. PULMONIFERA. Contains the air-breathing 

 Gasteropods, and to it consequently belong all the terrestrial 

 mollusca, though some few aquatic genera are also included. 

 The members of this order have an air-chamber instead of gills, 

 and are divided into two groups, (a) those without an operculum, 

 and (b) those having an operculum. Foremost in the first group 

 stands the great family 



I. Heliddce, named after 

 its chief representative, the 

 genus Helix. It also includes 

 the " Glass-shell "( Vitrina], 

 the "Amber-shell (Sucdnea), 

 and such genera as Buliimis, 

 Achatina, Pupa, Clausilia 

 (Fig. 13), etc., which differ 

 from the typical Helix in ap- 

 pearance, possessing as they 

 do comparatively high spires. 

 2. TheZ/;;z#czYfe,or "slugs," 

 follow next; of these only 

 one, the genus Testacella, has 

 an external shell stuck on the end of its tail ; the rest have 

 either a more or less imperfect shell concealed underneath the 

 mantle, or else none at all. 



3. The Oncidiadce are slug-like, and devoid of shell. 



4. The Limnceidce embrace the " Pond-snails," chief of whom 

 is the well-known, high-spired Limntea stagnalis. Other shells 

 of this family associated with Limnaa are, however, very 

 different in shape ; for instance, Physa has its whorls turning to 

 the left instead of to the right ; Ancylus (Fig. 24), or the fresh- 

 water limpet, is of course limpet-like ; while Planorbis, or the 

 " Coil-shell," is wound like a watch-spring. 



5. The Auriculida includes both spiral shells, such as Auricula 

 and Charychium, and a limpet-like one Siphonaria* 



At the head of group b stands I, Cydostomida. Cydostoma 



D 



Fig. 13. Clausilia biplicata. 



