230 PNEUMONOBRANCHIATA. 



snails and the Whirl-shells are left nearly dry by the 

 evaporation of the water, either by the heat, or by dry- 

 ness of the weather in winter, these animals assume 

 the character of terrestrial Mollusca, thicken and re- 

 flect their mouth, and form an epiphragm to prevent 

 themselves from being destroyed by the drying up of 

 the fluid necessary for their respiration and life. 

 Specimens in this state have been observed by Muller, 

 Maton and Racket, Montagu, Michaud, and others, 

 and have generally been considered as distinct 

 species. 



The family contains eight genera, which may be 

 thus distinguished : — 



I. Shell ovate, spiral ; pillar with an oblique plait. 



1. Limnceus. Shell rough; inner lip simple, (p. 

 219. f. 6, 7.) 



2. Amphipeplea. Shell polished, thin; inner lip 

 expanded. 



II. Shell conical, recurved ; apex oblique. 



3. Ancylm. Apex of the shell to the right, (p. 219. 

 f. 8.) 



4. Velletia. Apex of the shell to the left. 



III. Shell ovate, spiral ; pillar simple. 



5. Physa. Inner lip expanded, (p. 219. f. 9.) 



6. Aplexus. Inner lip not expanded. 



IV. Shell discoidal. 



7. Planorbis. Cavity of shell simple ; mouth round- 

 ish or subquadrate. (p. 219. f. 10.) 



8. Segmentina. Cavity of shell divided by cross 

 septa; mouth triangular. 



