COLOUR OF MOLLUSCA. 233 



formed; for these pearls are only formed on the Pinnae, 

 which internally are pink and semi-transparent, and the 

 black and purple specimens are generally more or less 

 opaque. 



" Their lustre, which is derived from the reflection of the 

 light from their peculiar surface produced by the curious 

 disposition of their fibres, and from their semi-transparency 

 and form, greatly depends on the uniformity of their texture 

 and the colour of the concentric coats of which they are 

 formed. That their lustre does depend on their radiating 

 fibres, may be distinctly proved by the inequality of the 

 lustre of the ' Colombian pearls,' which are filed out of 

 the thick part near the hinge of the Pearl-oyster (Avicula 

 margaritifera) ; so that they are formed, like that shell, of 

 transverse laminae, and they consequently exhibit a plate 

 of lustre on one side which is usually flat, and are sur- 

 rounded by brilliant concentric zones, which show the places 

 of the other plates instead of the even, beautiful, soft lustre 

 of the true pearls." 



3. COLOURS. The colouring of the shell is a part of the 

 theory of its formation, which will be more fittingly dis- 

 cussed hereafter ; but since their colours depend on the 

 secretion of a peculiar matter in the mucous skin, this may 

 be not an improper place to introduce some facts I have 

 collected relative to the colours of the animal considered 

 abstractedly, and which are principally intended to prove 

 to you that there is not any strict correspondency between 

 those of the tenant and its testaceous covering. Those 

 parts of the body of the mollusk which are constantly 

 covered are usually of a uniform white, a straw, or a greyish 

 colour ; and the dark spots with which they are clouded 

 are almost always occasioned by the opacity of the internal 

 viscera or their contents ; but the organs extended beyond 

 the shell, and which have felt the influence of the light, 

 are very often vividly tinted and variegated ; and you may 

 deem it very probable that the intensity of the colours will 

 be deepened or mitigated by the higher or lower latitude 

 of the shore inhabited by even the same species. Our 

 native Cowry (Cypraea europasa) is a plain white shell, but 

 its snail is a very elegant creature. The proboscis is dark 

 vermilion ; the tentacula yellowish red, spotted with yel- 

 low ; the upper part of the foot streaked longitudinally 

 with yellow and brown ; and the mantle greenish brown, 

 edged with brownish red; but, notwithstanding, the shell 

 is a uniform dull white. Similar discrepancies between the 

 colour of the shell and its owner are often met with : thus, 



