GASTEROPODA. LIMNADA. - 101 
each side most distinctly ; but in most cases they coalesce in 
pairs before they reach the ganglia, as is the case in Stagnicola, 
Gulnaria and Nauta. 
The sexual parts are distant from each other, which enables 
one individual, at the same time, to perform the function of 
each sex, with two others, as was first observed by Geoffroy 
about the middle of the last century. 
They all have the power of crawling on the surface of the 
water, their foot being uppermost, and parallel with the surface 
of the water. In this point of view, their mode of taking in 
air for respiration may readily be seen. I have occasionally 
observed Gulnaria and Nauta moving an inch below the surface 
of the water, in an inverse position. 
If when creeping at the bottom of the water another species 
impedes its progress, or creeps against it, the foot of the 
annoyed animal becomes a fixed point, and the offender is 
repulsed by repeated blows of its shell, which is whirled round 
like aclub. This is, I believe, their only mode of defence. 
The whole system, as well as the shelly covering, is sinistral 
or reversed, in the genera Physa, Nauta, Planorbis, and Hemi- 
thalamus ; whilst it is regular or dextral in the other genera, 
Stagnicola, Gulnaria, and Myzas. 
The shell is spirally elevated in all the genera but P/anorbis 
and Hemithalamus, where the whorls are involute, and conse- 
quently the shell is depressed. 
The last whorl in most of the genera and species of this 
family, in the old state, is subject to be set with facettes, re- 
sembling cut glass. This character has sometimes been used 
to distinguish species, by cabinet zoologists. 
Genus 45. STAGNICOLA. 
Lymnea, Lamarck. 
Limneus, Draparnaud, Férussac. 
Lymneeus, Cuvier. 
Testa spira valdé, subgradatim, elevata, conico-subulata. 
Umbilicus perforatus, clausus. Tentacula elongata. Pallium 
non dilatatum. 
Shell, with its spire very much and rather gradually elevated, 
