424 THE COLOUR OF SHELLS. 



The thinnest specimens of a species are generally the best 

 coloured. The light certainly exerts considerable influence 

 on the strength of colour, even in marine shells ; and it 

 appears to be owing to its modifying power that many 

 Nassas, Buccina, Naticse, Cypraeee, and other littoral shells, 

 have the back part much darker than the rest. This is 

 particularly the case with Nassa glans, Natica castanea, and 

 several Cowries, as Cyprsea stolida, C. errones, and C. caurica, 

 which have always an irregular bright bay spot on the back 

 of the body volution. 



The colour of shells is generally disposed in rays, streaks, 

 or bands, arising from the nucleus and extending to the 

 circumference ; in the spiral shells the lines of colouring 

 consequently follow the direction of the whorls. The rays 

 vary greatly in size; they are sometimes interrupted, and 

 they generally become wider as the shell grows larger. 



These coloured bands are evidently deposited by glands 

 placed on the margin of the mantle. Sometimes the action 

 of the glands is interrujjted, and the bands are broken. In 

 a few shells this suspension of the action of the glands takes 

 place at regular and very short intervals, in which case there 

 is formed a chain-like band, as in Marginella catenata, 

 certain Cones, and some other shells. In the Volutae, Olivse, 

 Coni and some Cassides, the colouring often forms angular 

 lines, so disjjosed, that the glands which deposit it seem to 

 have receded from each other, and then again contracted 

 together. Sometimes, as in Oliva tessellata, for example, 

 these lines are broken into spots ; but even in this species 

 some specimens exhibit the spots united into angular lines. 



In general the colour is situated on the outer coat of the 

 shell. It is often deposited on the inner side of the outer 

 layer, as in Strombus bitubercularis, and sometimes extends 

 a little into the outer j)art of the middle layer ; but I do not 

 recollect to have ever seen it pervade the whole thickness of 

 this coat. This circumstance leads me to believe that the 

 colouring matter is generally deposited by the glands imme- 

 diately after the deposition of the calcareous particles on the 

 periostracum, and during the formation of the outer coat 

 which, as will be seen hereafter, is always deposited before 

 the two inner ones.* 



This situation of the colouring matter explains the reason 

 why many shells, such as the Olives and Cones, are darker 

 when their outer coat is removed ; this is particularly the 



* Sometimes the colour coat is destioyed by age : thus in tlie old Cowries 

 the colour is obliterated by a gaudy olivaceous coat. 



