198 LIMNJEAD.E. 



either by the heat, or by dryness of the weather in 

 winter, these animals assume the character of ter- 

 restrial Mollusca, thicken and reflect their mouth, 

 and form an epiphragm to prevent themselves from 

 being destroyed by the drying up of the fluid neces- 

 sary for their respiration and life. Specimens in 

 this state have been observed by Miiller, Maton and 

 Racket, Montagu, Michaud, and others, and have 

 generally been considered as distinct species. 



The family contains nine genera, which may be 

 thus distinguished : 



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



1. Limnaus. Shell rough; inner lip simple. 



(fig. 48.) 



2. Amphipeplea. Shell polished, thin ; inner lip 



expanded. 



b. Shell conical, recurved ; apex oblique. 



3. Ancylus. Apex of the shell to the right. 



(fig. 52.) 



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



c. Shell ovate, sub-spiral ; pillar smooth. 



5. Otina. 



d. Shell ovate, spiral ; pillar simple. 



6. Physa. Inner lip expanded. 



7. Aplexus. Inner lip not expanded. 



e. Shell discoidal. 



8. Planorbis. Cavity of shell simple; mouth 



roundish or subquadrate. 



9. Segmentina. Cavity of the shell divided by 



cross septa ; mouth triangular. 



