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 : 



1. Streptoneura, in which the torsion of the 

 visceral loop is well marked. 



2. 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 — 



