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 aperture of the shell, generally spoken of as the mouth 

 peristome, or perttreme, 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, 

 (born 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 Bulimus 

 decollatus, 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 size. 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 the molluscan 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 sinistral or laotrope 

 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 not 

 rare ; and green infrequent. Some shells are unicolorous and not 



