24 LAND AND FRESH-WATER SHELLS. 



Helix (H. rupestris, H. pygmea, H. rotund at a, H. 



pulchella\ Carychium, P/iysa, Planorbis (excluding 



P. corneus). 

 4. Odontognatha. Ribbed, margin denticulated or crenu- 



lated. 

 (a) Slightly arched, ribs numerous, bluntly denticulate 



margins. Arion, Geornalacus , Helix (//. obvoluta. If. 



cantiana, H. cartusiana, H. sericea. H. hispida, H. 



concinna, H, rufcscens, H. acuta). 

 (4) Strongly arched, ribs few. Helix (H. ericetoruin, 



H. pisana, H. caperata, H. virgata, H. nemor alis. 



H. horlensis, H. aspersa, H. pomatia). 



III. TRIGNATHA. Mandibles three. Limnaa^ Ancylus, Planorbis 

 corneus. 



IV. DIGNATHA. Mandibles two, lateral. Cydostoma, Acme, 

 Paludina, and the other fresh-water Gastropods. 



The radula consists of a chitinous plate beset with teeth, which 

 as they are worn away are replaced by others from behind. 

 Taking a transverse row of teeth, it will be found that they vary 

 in different places, and it is possible to represent them by using a 

 formula. " Thus, when in each row there is a single median tooth 



Fig. 7. A segment of the radula of Neritinafluviatilis (after S. Loven). 



with three teeth on each side of it, more or less closely resembling 

 one another, we write the formula 3.1.3. When there are addi- 

 tional lateral pieces of a different shape to those immediately 

 adjoining the central teeth, we indicate them by the figure o, 



