132 
NATURAL HISTORY. [EAST, ZOOL, 
transparent, 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, and 
forms a membranaceous epiphragma over the mouth, like 
the snails. The shells are generally external, as the Limncece 
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 discoida! 
depressed shell, with the whorls revolving one over the 
other on their own axis. The Physce bear the same rela¬ 
tion to the Aplexi as the Arnphipeplece to the Limnceoe? 
but they both differ in the edge of the mantle being pro¬ 
duced and reflexed over their thin polished shell when the 
animal is expanded. The three first genera have an upper 
and two lateral jaws, and simpleconical teeth on the tongue, 
w T hile the two latter have only a single upper jaw and ser¬ 
rated teeth. The Ancyli only differ from the Pond Snails 
in having a simple conical shell, which has caused 
them to be called fresh-water Limpets. The Velletice 
differ in the animal and shell being reversed, like the 
Physce. The animals of Segmentina , which are like the 
Planorbes , deposit at each stoppage in their growth three 
transverse plates, which contract the mouth of the shell, 
and make it appear as if it was chambered; hence it has 
been called a fresh-water Nautilus . 
The marine lung-breathing Mollusca have no distinct 
tentacles, as the three following families: — 
The family Amphibolidce have a subglobose, spiral, urn- 
bilicated shell, with an oval mouth and a rather expanded 
outer lip, which is sinuated behind ; the head has no 
tentacles, but a transverse disk across the front, which is 
free at top, and has the eyes placed on its outer hinder 
side. The operculum is horny and spiral. They are only 
found in the Pacific Ocean. 
The family of Siphonariadce are like the former, but 
they are protected by a conical shell like a Patella , with 
which they have been confounded ; but the apex is rather 
on one side and posterior, and the scar left by the adductor 
muscle near the edge of the cavity (as in Ancylus) is 
interrupted in the centre of the right side, where the 
breathing-hole of the dorsal respiratory cavity is placed; 
they have no operculum. 
