24 LAND AND FRESH-WATER SHELLS. 



Helix (H. rupestris, H. pygmea, H. rotundata, Jf. 

 pulchelld], Carychium, Physa, Planorbis (excluding 

 P. corneus). 



4. Odontognatha. Ribbed, margin denticulated or crenu* 

 lated. 



(a) Slightly arched, ribs numerous, bluntly denticulate 

 margins. Arion, Geomalacus, Helix (H. obvoluta, H. 

 cantiana, H. cartusiana, H. granulata^ H. hispida, H. 

 conrinna, H. rnfescens^ H. acuta). 



(b) Strongly arched, ribs few. Helix (H. if a/a t H. 

 pisana y H. caper ata, H. virgata, H. minor alis> H. 

 hortensiS) H. aspersa, H. pomatid). 



III. TRIGNATHA. Mandibles three. Limncea, Ancylus, Plan- 

 orbis corneus. 

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



Vivipara, and the other fresh-water Gastropods. 

 The radula consists of a chidnous 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" 



p^ 7. () A segment of the radula of Pterotrachea !esiie>-ii (after Macdonald) ; (6) ditto 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 



