1 8 LAND AND FRESH-WATER SHELLS. 



an elastic ligament about half a whorl from the aperture. It 

 serves exactly the same function in the economy of the animal 

 as an operculum does among the Paludinidae and Cyclostomatidae. 

 When the animal extends itself out of the shell the clausilium is 

 pushed back against the columella, and when it withdraws, the 

 clausilium flies backwards on account of the elasticity of its 

 ligament, and closes the mouth of the shell. 



The peristome varies greatly in shape in different species. 

 When it is entire the shell is called holostomatous ; when produced 

 into a canal for a siphon it is spoken of as siphonostomatous. 

 It is sometimes reflected over the umbilicus ; this is due to 

 the presence of a lobe in the collar of the mantle of the animal, 

 termed the columellar lobule. It was John Hunter, the great 

 anatomist (bom 1728, died 1793), who first noticed that the 

 animal has the power of absorbing part of its shell, and this 

 will explain the thinning of some parts of a whorl in a shell 

 in relation to the other portions, as is sometimes observed. 

 In some species, as Stenogyra decollata, the apex becomes, as 

 the animal grows to the adult condition, decollated; i.e., the 

 physical decomposition of the apex of a shell, due to the animal 

 leaving it for the lower and larger whorls as it grows in srze. 

 When a shell is injured at the peristome all the layers are 

 reproduced ; when at any other portion only the middle and 

 internal layers are repaired, and then the shell is generally 

 thickened internally. 



The spire of the majority of our land and fresh-water shells 

 turns to the right, and the mouth, when placed in its proper 

 position, looks towards the left ; it is then spoken of as dextral 

 or dexiotrope. In some genera, as Physa, Balia, and Clausilia, 

 the reverse of this obtains, and then the shell is said to be sinistnil 

 or Idotrope the spire turns towards the left, and the mouth, 

 placed in its proper position, looks towards the right. 



The colour of the shell varies in different species, and often 

 in the same species. Camerano has studied the comparative 

 rarity and frequency of the colours of the shells of the Mollusca, 

 and his inferences are that black is rare; brown, grey, yellow, 

 white, and red common ; violet relatively abundant ; blue noi 

 rare; and green infrequent Some shells are unicolorous and not 



