104 
NATURAL HISTORY. [NEW BUILDING. 
having cylindrical tentacles, like the land sings, 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 Scarabus, which has these parts only incompletely 
developed. The true Auriculae 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. Cari- 
chium is one of the smallest British shell-bearing Mollusca; 
it has a sinuated mouth and a reflexed lip, like a Bulimus . 
The Conovuli , 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. 
6. The family of Pond Snails (Limnceadte, Case 26) 
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 vegetables, having a 
muscular stomach. Like many marine Gasteropodes, 
they have the power of floating on the surface of the 
water, with the back downwards, the concave surface 
of the foot forming a kind of boat; their eggs are trans¬ 
parent, and deposited in oval masses on water-plants. 
The outer lip of the shell is thin, but when the ponds are 
dried up, the animal strengthens it by an internal rib, ane 
forms a membranaceous epiphragma over the mouth, likd 
the land-snails. The shells are generally external, as the 
Lhnncece and Aplexi; the former having an oval shell and 
triangular tentacles, and the latter filiform tentacles and 
a reversed shell, like the Planorbes , which differ in having 
a discoidal depressed shell, vrith the whorls revolving 
