MOLLUSCA. GASTEROPODA 237 



7. Globular; as in Natica. 



8. Convolute; when the last whorl covers all the 

 others and the aperture is consequently as long as the 

 shell, as in Cyprcea. 



9. Auriform; aperture very large and spire very 

 short, as in Haliotis. 



The surface of the shell is frequently ornamented with 

 spines, knobs, ribs, or striae; these are said to be spiral 

 when they run parallel with the sutures, and transverse 

 when they run across the whorls from suture to suture. 

 In some genera {e.g. Murex) rows of spines, or lamellar 

 processes, extend across all the whorls from the apex to the 

 base of the shell, forming what are termed varices. The 

 surface of the shell in recent gasteropods is generally 

 coloured, often variegated ; in fossil examples the colour 

 has nearly always disappeared, but a few specimens, from 

 various formations, even as early as the Carboniferous, 

 have been found showing the colour more or less perfectly 

 preserved. The shell consists of an outer chitinous layer, 

 and of a calcareous layer which is thick and porcellanous ; 

 in some cases there is also an inner nacreous or pearly 

 layer. 



The Gasteropoda are divided into two sub-classes : — 

 (1) Isopleura, (2) Anisopleura. 



SUB-CLASS I. ISOPLEURA 



The Isopleura, or Polyplacophora, include Chiton and 

 its allies. The body is bilaterally symmetrical and more 

 or less elongated, with the anus at the posterior end. 

 There are numerous (6 to 80) pairs of gills, which are 

 placed in a groove between the foot and the mantle. A 

 nerve-ring surrounds the oesophagus, and from it two 



