4 The ShellCollectons Handbook. 
shaped according to the contour of the mantle edge, 
and is eeerunnded by a lip or perdtreme. It may be 
entire as in the phytophagous species, or prolonged 
into siphons as in the carnivorous species. 
If the spiral of the shell be turned to the left with 
the mouth towards the right, the shell is said to be 
sinistral or ladtrope, and if to the right with the 
mouth to the left dextral or dexiotrope. Each suc- 
cessive growth of the shell is represented by lines— 
lines of growth—arranged concentrically with the 
nucleus. In the Limacide the shell is rudimentary, 
and situated under the mantle. 
The colour of the shell exists entirely in the 
pertostracum or epidermis, and Camerano (Richerche 
intorno alla distribuzione dei colori nel regno animale. 
Mem. Acc. Tor. xxxvi. pp. 329, 360), finds that of all 
colours black is rare ; brown, grey, yellow, white and 
red common; violet relatively abundant; blue not 
rare, and green infrequent. 
Beneath the epidermis is a densely calcified layer 
(honeycomb layer), showing a confused striated super- 
ficial portion, and a deeper portion of vertical prisms. 
Below this again is the nacreous or mother of pearl 
layer, consisting of thickly calcified lamin placed 
upon one another, with the axes of the plates, in 
successive layers, set at right angles. 
In some species the aperture of the shell is closed 
normally by a horny or shelly plate (operculum) which 
differs from the true shell in having more conchiolin 
entering into its composition. In non-operculated 
‘land shells, during the winter months the mouth is 
closed by a layer of mucus impregnated with cal- 
