Fig. 13. Clausilia biplicata.
2. The Limacidæ, 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 Oncidiadæ are slug-like, and devoid of shell.
4. The Limnæidæ embrace the "Pond-snails," chief of whom is the well-known, high-spired Limnæa stagnalis. Other shells of this family associated with Limnæa 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 freshwater limpet, is of course limpet-like; while Planorbis, or the "Coil-shell," is wound like a watch-spring.
5. The Auriculidæ includes both spiral shells, such as Auricula and Charychium, and a limpet-like one Siphonaria.
At the head of group b stands 1, Cyclostomidæ. Cyclostoma elegans is a common shell on our chalk-downs, and well illustrates its family, in which the mouth is nearly circular. Foreign examples of this genus are much esteemed by collectors. The other two families are, (2) Helicinidæ and (3) Aciculidæ.
