XXIV INTRODUCTION. 



are reflected back on its convexity, and cover it so 

 completely, that not an atom of the shell appears. 



These two wings (which are independent of the 

 collar of the animal) are covered with glands con- 

 taining colouring liquors peculiar to themselves ; 

 and it is their upper surface, or that which is laid 

 against the convexity of the shell, that alone fur- 

 nishes the secretion. Whence it follows, that these 

 wings deposit new layers of testaceous matter, 

 differently coloured from those beneath, and diver- 

 sified with spots, either entire, round, or zigzag. 

 Their variety of colour is very great ; they are 

 sometimes white on a brown ground, or tawny 

 on a yellow ground, or yellowish on a tawny ground. 

 Sometimes we meet with lines, instead of spots, 

 strait, curved, or reticulated ; of different shades, 

 on grounds of various colours ; and occasionally the 

 surface is covered with points (or dots), disposed in 

 every direction. 



We have observed, while speaking of the varie- 

 gated helix, that its shell was subject to remark- 

 able varieties of colour, depending on the different 

 form and arrangement of the secreting organs. 

 The changes which these organs undergo, and every- 

 thing that relates to their oeconomy , will apply equally 

 to all the marine, fresh water, and land snails, that 

 exist in nature. But to the causes already de- 



