GALLERY. 3 NATURAL HISTORY. 131 
of the mantle. The Onchidia live on aquatic plants in 
ditches, and Onckis and Peronia under stones on the sea¬ 
shore. 
2. The aquatic kinds, on the contrary, have the head and 
tentacles of the same structure and surface as the rest of 
the body, and like it, are only contractile; the eyes are 
always placed on the side of the base of the tentacles. 
They are divided into two families: the Auriculidce and 
Pond Snails, or Limnceadce. 
The family of Auriculidce are peculiar for having cylin¬ 
drical tentacles, like the land slugs, and their eyes are 
placed on the inner side of the base of these tentacles. They 
have a ringed conical muzzle. The mantle is thin, with a 
thickened edge ; they are always provided with an external 
spiral shell, which has a plaited pillar in all ages; and the 
animal has the peculiarity of absorbing the septa which 
separate the cavities of the whorls from one another, even 
in Scarabns, which has these parts only incompletely deve¬ 
loped. The true Auriculce have a thickened edge to the 
mouth of the shell, which is covered with a brown perios- 
traca. The Scarabus, like Ranella , forms half a whorl be¬ 
tween each period of rest, the thickened and reflexed part 
of the lips forming an edge to each side of the shell. The 
Sidulce have a sharp internal ridge to the outer lip. Card 
chium is one of the smallest terrestrial Mollusca; it has a 
sinuated mouth and a reflexed lip, like a Bulimus. The 
Conomdi , which are found under stones on the sea-shore, 
and in the mud of salt marshes, have an obconic shell 
with a narrow linear mouth; and the Chilince, which live 
in clear running streams, in South America, have much 
the habit of the Pond Snails, from the shells of which 
they are chiefly to be distinguished by the sharpness of the 
plaits on the pillar, and by the shell being spotted. 
The family of Pond Snails ( Limnceadce ) differ from the 
former in having compressed tentacles, with the eyes at the 
outer side of their base; their muzzle is short and dilated 
at the end ; their shell is uniformly horn-coloured, with a 
more or less oblique plait on the pillar; they live on vege¬ 
tables, having a muscular stomach. Like many marine 
Gasteropodes, they have the power of floating on the sur¬ 
face of the water, with the back downwards, the concave 
surface of the foot forming a kind of boat; their eggs are 
