INTRODUCTION. XXI 



colour, and others marked with numerous spots, often 

 of an irregular shape, and separated from each other 

 at unequal intervals. This is the case (among other 

 examples) with the black tiger cone (Conus mar- 

 moreus, Lirm.) In this, as well as in the helix, 

 however, the spots are formed by the collar, and 

 the black pigment secreted at intervals, so as to form 

 irregular stains, instead of uninterrupted bands. 



It must be observed, that the interior layer of the 

 shell, or that which covers the body of the animal, 

 independent of its collar, ought to be white, since 

 the secretion, as we have noticed already, is gene- 

 rally colourless. It is sometimes, however, other- 

 wise ; the body of the animal will exude a coloured 

 liquor, and then the interior of the shell will be 

 stained accordingly ; but the tint will be uniform, 

 and never varied like the exterior. To make the 

 different operations of the animal perfectly apparent, 

 let the outer coat of a shell be filed off, and the 

 layers beneath, which have been furnished by the 

 body, will be uniformly white, or tinted; while the 

 exterior, which belongs exclusively to the collar, will 

 be variegated. 



It will here be necessary to make some further 

 observations on the growth of shells, in order to ac- 

 count for the change of colour which we sometimes 

 perceive in the bands of univalves. The different 



