105 
EAST. ZOOL. GAL.] NATURAL HISTORY. 
one over the other on their own axis. The Physce bear 
the same relation to the Aplexi as the Ampliipeplece to 
the Limnceaz, but they each have the edge of the mantle 
produced and reflexed over their thin polished shell when 
the animal is expanded. The three first genera have an 
upper and two lateral jaws, and simple conical teeth on the 
tongue, 'while the two latter have only a single upper jaw 
and serrated 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 ; and Discus has 
teeth in the cavity of the shell. 
C. The marine lung-breathingMollusca have no distinct 
tentacles, as the three following families: — 
7- The family Amphibolidce (Case 26) have a subglobose, 
spiral, umbilicated 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. 
8. The family of Siphonariadce (Case 26) 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. 
9. The family of Gadiniadce (Case 26) have an animal 
and shell like the former, but the passage to the respiratory 
cavity is in front of the adductor muscle, its place being 
marked in the cavity of the shell by a slight groove. 
II. In the remainder of the animals of this order, the 
respiratory chamber is open, that is to say, the front edge 
of the mantle is free from the'back of the neck, leaving a 
large slit for the admission of the air into the bag. They 
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