EXERCISE VIII. 
THE SEA SNAIL (Amphibola avellana). 
Sub-kingdom—MOo.Luusca. Class—GASTEROPODA. Order—PULMONATA. 
1. External characters.—The general form of the shell 
is sub-globose, with a very short spire. On its basal 
surface there is a hole, the wmbilicus. The aperture 
is semicircular, and on its inner side the columella is 
flattened and reflected over the umbilicus; the outer 
lip of the aperture has at its upper or posterior end a 
channel or sinus. The whorls are angled and 
flattened behind, and are transversely rugosely lami- 
nated, each of the laminze marking a period of growth 
of the animal. Externally the shell is yellowish or 
reddish brown, generally more or less purple on the 
spire and keel ; the interior is brownish purple, with 
a white rim round the aperture. The animal is very 
sluggish ; it is ofa dark purple, with a lighter coloured 
foot. The pulmonary opening is on the right side 
just under the posterior sinus of the shell. On the 
head the frontal lobes are emarginate, and on each 
side is a small flattened triangular tentacle with 
minute sessile eye at its base. On the back of the 
foot is a horny operculum. 
2. Internal structure—Break the shell by a gentle 
blow with a hammer. The animal will be found to 
be attached to the columella of the shell by the 
