CLASSIFICATION 23 
by the genus of land shells called Clausilia (Plate 
VIII., Fig. 17). The “clausium (c),” as it is called, 
is a spoon or shoehorn shaped elastic plate, attached 
by its stalk to the columella within the shell, and 
closes automatically behind the retiring occupant. 
One remarkable little Land Snail (Thyrophorella), 
inhabiting the island of St. Thomas, in the Gulf of 
Guinea, is said to be able to close the mouth of its 
shell by bending down a portion of the peristome 
which projects beyond the upper half of the aperture, 
and is furnished with a hinge for the purpose 
(Plate VIII., Figs. 18 and 19). 
In many Gastropods, especially the Land Snails, 
the ingress of larger enemies is barred by projecting 
teeth placed round the aperture, or, less frequently, 
some way back in the shell. Sometimes these teeth 
are so long and numerous that it is a matter of 
wonder how the rightful owner itself gets in and out 
of its house (Plate VIII., Figs. 14-16). 
The Gastropoda are divided into two sub-classes : 
I. STREPTONEURA, in which the torsion of the 
visceral loop is well marked. 
g. EUTHYNEURA, in which, in the adults, owing to 
partial detorsion of the visceral hump, the visceral 
loop nearly becomes once more simple. 
Sub-Class 1: The STREPTONEURA (also known as 
PROSOBRANCHIA, because the gills are in advance 
of the heart), primarily by the gill structure and 
secondarily by the radula, are divided into— 
