198 
LIMNiEAD^. 
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 mouthy 
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 Muller, 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. Limnceus. Shell rough; inner lip simple. 
(fig. 48.) 
2. Ampliipeplea. Shell polished, thin ; inner lip 
expanded. 
h. 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. Planorlis, Cavity of shell simple ; mouth 
roundish or subquadrate. 
9. Segmentina. Cavity of the shell divided by 
cross septa ; mouth triangular. • 
