SLUGS AND SNAILS. 273 



insects. The most common of these has a chocolate- 

 coloured shell, with a white lip, and a single tooth at 

 the mouth. 1 It is a very minute shell, scarce more than 

 one-eighth of an inch long, and half as thick, found 

 amongst leaves and in the fissures of oak -bark, often 

 clustered together in considerable numbers. 



The last group of shells with which we need trouble 

 our reader are termed in science Clausilia^ because 

 the throat of the shell is closed over the retreating 

 animal by a palate, or kind of lid, called a dausium, 

 which is thus described by Gray : " It consists of a 

 spirally-twisted shelly plate, inclosed in the last whorl 

 of the shell When the animal is retracted within its 

 shell this shelly plate nearly covers the aperture at a 

 little distance within the mouth, and coming in con- 

 tact with a transverse plait on the outer lip, leaves 

 only a small canal formed between the outer plait and 

 the posterior angle of the mouth, and sometimes an 

 elongated longitudinal plait on the inner lip. When 

 the animal wishes to protrude itself, it pushes the plate 

 on one side into a groove, situated between the inner 

 plait and the columella, where it is detained by the 

 pressure of the body of the animal, leaving the aper- 

 ture free ; and when the animal withdraws itself, the 

 plate springs forward by the elasticity of the pedicel, 

 and closes the aperture." 



The first of these is the largest, often three-quarters 

 of an inch long, found amongst decaying beech-leaves, 

 especially in the south. 3 It is generally of a brownish 



1 Bulimus obscurus. 3 Clausilia laminatd. 



9 Pupa umbilicata. 



