INTRODUCTIOX. 15 



The terms Conchology and Testaceology, which are syno- 

 nymous, refer to all animals which have a testaceous or 

 shelly covering, whether they inhabit the sea, the land, or 

 fresh- water. 



Some chemical writers have divided shells into two classes. 

 The first are usually of a compact texture, resembling por- 

 celain, with an enamelled surface, and are in general beauti- 

 fully variegated. Those composing this class are called 

 porcelaneous shells, of which kind are those of the genera 

 Conus, Cypraea, Voluta, &c., of the Linnsean arrangement. 



The second class consists of shells generally covered with 

 a strong epidermis or skin, below which lies the shell in 

 layers, and composed entirely of the substance well-known 

 by the name of mother-of-pearl. In this class are the fresh- 

 water Muscles, (the Mytilus Cygneus and Anatinus of Lin- 

 nffius,) the genus Haliotis or ear-shells, the genus Turbo, 

 and several of the Trochus tribe, &c. 



The porcelaneous shells contain a very small portion of 

 soft animal matter, and those of the second class a very 

 large proportion ;* but otherwise there is no real distinction 

 between them. 



Shells, then, are only the habitations of soft animals called 

 Vermes, or Worms, which constitute the third order of 

 Linnaeus's sixth class of animals. According to that author, 

 the animals which inhabit them may be arranged into ten 

 genera, as follows : 



1. Limax; 2. Doris; 3. Spio; 4. Amphitrite ; 5. Tere- 

 bella; 6. Nereis; 7- Ascidia; 8. Tethys; 9. Triton; 10. 

 Sepia. 



1. I-IMAX — Body oblong, creeping, with a fleshy kind of 

 shield above, and a longitudinal flat disk beneath ; aperture 

 placed on the right side, within the shield; feelers four, 

 situate above the mouth, and an eye at the tip of the largest 

 ones. 



This genus is what is generally termed the slug, or snail. 



2. Doris — Body creeping, oblong, and flat beneath; 

 mouth placed below, on the fore-part; vent behind, on the 



* See Thomson's System of Cheinistry, vol. v. p. 507. 



b2 



