92 GASTEOPODA. 



from east to south. Accidental specimens are 

 occasionally found, which turn in the opposite 

 direction, termed sinistral, or reversed, and in a 

 few instances this is the natural figure of the 

 species. The axis around which the spire is 

 rolled may be hollow, when the shell is umbili- 

 cate ; or solid, when it is pillared. That edge of 

 the orifice which is formed by the margin of the 

 shell is the outer lip ; that which is formed by the 

 pillar is the inner lip. 



In the carnivorous GASTEOPODA^ the base of the 

 shell is grooved to form a canal, through which 

 the proboscis is protruded ; but the canal may be 

 merely a notch, or lengthened into a long gutter. 



The shell is formed in the same manner as in 

 the CONCHIFEEA, by the folding back of the edge 

 of the mantle. This edge in some species has 

 projections which secrete shelly matter, producing 

 spinous processes, ribs, knobs, or other irregu- 

 larities on the surface or on the margin of the 

 shell. Occasionally the two sides of the mantle 

 are bent upwards, embracing more or less com- 

 pletely the shell, in which case the exterior is 

 covered with a glassy, porcelain-like coat ; this is 

 the case with the well-known and beautiful Cowries 

 ( Cyprceadce) . 



Very many species carry on the hinder part of 

 the body a horny or shelly plate (operculum), which 

 accurately closes the aperture of the shell, when 

 the animal has withdrawn into its recesses. The 

 form of this appendage is ordinarily that of a very 

 flat cone, made by successive layers, each a little 

 larger than its predecessor, or a flattened spire. 

 The common Top- shells (TrocJius) afford good 

 examples of the spiral operculum. 



