98 



SCIENCE- G OSSIP. 



the underpart surrounding the opening of some 

 shells belonging to the genera AporrJiais and Conns, 

 is insoluble, although the upper portions yield the 

 usual blue with nitric acid. Sometimes nitration 

 has already taken place naturally, and then we get 

 violet shells, such as that of Ianthina communis, 

 whilst the shell just inside the opening is often of 

 this colour. Reginula araclinoides being a good 

 example. The purple dye produced by many 

 Muricidae and Buccinidae may be of the same 

 nature. Even this form of colouring is probably 

 protective. Thus I. communis is pelagic in habits. 

 and its bluish colour must cause it to be very incon- 

 spicuous when floating on the sm-face. Most species 

 of Do/iax are covered by a coloured periostracum. 

 which skin is insoluble in acids, but nitric acid 

 will soon break through it and stain blue the true 

 colouring matter underneath. On a few species, 

 such as D, reticulata (fig. 1. Xo. 8), the covering is 

 absent. 



No terrestrial or fluviatile shells are coloured by 

 iron, but most, including those of the genera 

 Helix and Iliilimus, from all parts of the world, 

 except Australia, by the second, blue-yielding 

 compound. That in Planorhis, Cyelostbma. and 

 Helicina (fig. 1) does not turn blue, thus being 

 analogous to the third form in marine shells ; 

 whilst the colour of those of the genus TJnio 

 is caused by an insoluble brown or green 

 periostracum, which is stained yellow by nitric 

 acid. The pigment of all the Australian ffehces 

 I have as yet examined does not turn blue 

 — H. frinff'lla is a typical example — although 

 that in South American species does so. This is 

 interesting if. as I believe to be the case, the in- 

 variable form is the elder of the two organic pig- 

 ments. Specimens from South America are usually 

 found covered with a dark brown periostracum, 

 which must of course be removed by softening 

 with an acid and then peeling off before testing 

 the colour. The absence of iron in land and fresh- 

 water shells is very natural, for they obviously 

 have not the same opportunity to secrete it as 

 marine species. It will he seen that the power of 

 depositing an organic pigment must have been 

 acquired independently by different genera, for 

 though some bivalves are coloured in this manner, 

 many univalves are still marked by iron, and a 

 number of allied forms differ in the nature of 

 their colouring matter. It must also be of great 

 antiquity, as land shells have been found in car- 

 boniferous strata, and these themselves were the 

 descendants of amphibian and fresh-water species. 

 2. I think it will not be disputed that the ob- 

 ject of coloration is protection, as the habit of 

 producing an organic pigment would hardly have 

 been formed without a purpose, and its independent 

 acquirement by different types of shells points to 

 its being of great utility to them. That the colour 

 is protective is the most reasonable explanation of 

 this, and indeed it is not difficult to imagine that 



tints usually ranging from dark brown to yellow 

 harmonise with the sea-floor on which the shells 

 rest. Even the tropical shells, that seem to us 

 so conspicuous, must, when lying on a variegated 

 pebbly or sandy bottom among similarly coloured 

 algae, be distinguished with difficulty by the 

 creatures from which they desire protection. That 

 there is a specific purpose in thus adorning their 

 shells — among, at any rate, those molluscs which 

 form an organic pigment— is shown by the fact that 

 none of the colouring matter is placed where it 

 would not be seen. All is deposited as a thin 

 coating on the outer surface of the shell ; none 

 penetrates the lime, and none is found on the 

 inside of the valves. On the other hand, with 

 species coloured by ferric oxide the iron not only 

 impregnates the shell, but is often more thickly 

 deposited inside than out. To illustrate the above 

 compare the interior of an oyster or mussel with 

 that of a tropical bivalve. 



3. The obvious answer to the question " From 

 whence is the colouring matter derived ? " is 

 " From the food of the mollusc " : and this, no 

 doubt, is in most cases correct. I think, however, 

 that a more rational explanation of the colours of 

 those shells stained with iron is as follows : — During 

 the growth of the shell a fresh layer of cells is 

 being continually exposed to the action of the sea- 

 water, and any particles of iron which were sus- 

 pended in the latter would very likely be deposited 

 in them. Also, if the mollusc had some control 

 over this deposition, it would be advantageous to 

 have any surplus on the interior, and thus prevent 

 the shell from becoming conspicuous by getting 

 too dark. There is an abundance of iron on the 

 sea-bottom, especially near the coast, where most 

 shells live ; every pebble and most sand are stained 

 with it : all mud contains some, and it is found 

 colouring many fossils in an irregular manner, 

 which shows that it was derived after the death of 

 the animal, from the sediment that gradually 

 enveloped the shell. 



Supposing, as seems probable, that iron was the 

 original colouring matter on primitive shells, there 

 are three reasons which might cause it to be 

 exchanged for another : — («) In the case of land 

 and fresh-water shells, absence of the means of 

 obtaining sufficient iron to furnish the depth of 

 colour required. (//) The desirability of acquiring 

 varied markings, which could hardly be formed by 

 the mechanical deposit of grains of iron oxide ; an 

 organic fatty matter is evidently much more suit- 

 able. Even then many variegated shells have 

 spikes or other projections to which the colour is 

 often confined, and which undoubtedly assist the 

 accuracy of the patterns and prevent the colour 

 from spreading. It would, however, be wrong to 

 consider the chief use of spines on shells was for 

 this purxoose, as many that possess them are of a 

 uniform tint — e.g. JIurex tenuispina — the left- 

 hand shell on the block heading the " Notes and 



