GASTEROPODA. 79 
The part upon which the spire turns is called 
the pillar. It is sometimes solid, but sometimes it is 
hollow ; in the latter case the perforation is named 
the umbilicus. When the spire is long, the shell 
is said to be turbinated, which is the common form ; 
but in some genera, as Planorbis, the convolution 
takes place in the same plane, and the shell is flat, 
or even concave. Such shells are termed discoid. 
When the upper part of each turn or whorl envelopes 
or covers that which preceded it, the spire is said 
to be concealed. 
In almost all species the convolution is towards 
the right side. There are a few, however, which turn 
to the left; these shells are termed reversed. The 
end of the latest whorl, where the animal protrudes, 
is termed the mouth or aperture. In order to close 
this, when the animal withdraws itself into its 
shell, the hinder part of the foot is usually furnished 
with a horny or shelly plate, called the operculum, 
which, when the animal contracts, is brought into 
such a situation, as more or less completely to close 
the mouth of the shell, when the animal is drawn 
into its cavity. It has hitherto been observed only 
in those Mollusca which have pectinate branchiz, 
and in two genera, Cyclostoma and Helicina, among 
the air-breathing land-shells. The form of the 
operculum is in general either that of a very low 
cone, made by successive layers, each one a little 
larger than its predecessor, or that of a flattened 
spire, and the texture is either horny or shelly. 
The species of Gasteropoda are very numerous, 
and are arranged in five orders, viz.—Pulmonifera, 
Nudibranchiata, Tectibranchiata, Cyclobranchiata, 
and Pectinibranchiata. 
