146 



SCIENCE- GOSSIP. 



C&ritlwum eoMnoides also possessed an organic pig- 

 ment. Washing with a dilute acid often brings 

 out the colour wonderfully, revealing the existence 

 of former markings on shells apparently quite 

 white. In this manner I have found that the 

 following shells were coloured, usually by bands, 

 like many of the same genera at the present day : — 

 Cytherea laevigata, Sunetta semi-sulcata, Pectiin- 

 cilkis pulvinatiis, and Corbula gallica. The latter 

 had violet bands comparable to those on the living 

 shell Venus fasciata. But the most interesting 

 example of this kind is Cardita plafrricosta. Shells 

 of this species, from the Paris basin at all events, 

 show, when developed with hydrochloric acid, a 

 system of bands, the positions of which on a 

 medium-sized specimen I have shown in fig. 3. In 

 young individuals the bands marked a and b, c and 

 d, and e,f, and g were joined together so as to form 

 three broader stripes, whilst on larger specimens 

 they are sometimes still further subdivided. It 

 would thus appear that bands marked the positions 

 of most rapid growth of the shell. Many fresh 

 cells were formed along their median lines, and 

 thus they tended to break up with age. It is quite 

 possible that this mollusc — which had a very wide 

 range, from the Mediterranean to North America — 

 may not have been coloured in the same manner 

 in all localities, so that if living to-day it would be 

 divided into separate varieties, if not species. Very 

 few French Eocene fossils were coloured by iron ; 

 the only ones I have found to have been so are 

 Lueina coneentrica, Lueina sulcata, Ostrea muta- 

 Ijilo, Cliama ealcarata, and Potamides emarginatiis. 

 All except the last, which probably lived in a 

 muddy estuary, are darkest on the inside ; the 

 exteriors of the two Lueinae were quite white. As 

 will be seen from the example figured (fig. 2, 

 No. 2), the colouring matter of these latter formed 

 an oval spot covering the whole of the interior of 

 each valve up to near the margin, and was orange 

 in colour, whilst an examination of a few speci- 

 mens would convince anyone that it was not 

 accidental, but really formed the pattern on the 

 shell. It gives a very light blue with potassium 

 ferrocyanide, so faint as to raise a doubt that it 

 was really iron. A mixture of iron persalts and 

 protosalts would, however, give this result, and 

 that this is what it is can be confirmed by testing 

 a solution in hydrochloric acid with caustic potash, 

 which, as on this hypothesis it should do, will 

 form a green precipitate. Now limestones and 

 shells originally white, from the Paris basin, are 

 often stained deep yellow by a substance which 

 has exactly the same properties as the above, and 

 must therefore be a mixture of oxides of such a 

 kind that with . an acid it always forms both per- 

 and protosalts. The importance of this lies in the 

 evidence it affords that the colouring of shells by 

 iron is entirely mechanical, since a particular 

 mixture of oxides is unaffected by being used on 

 the shell; the mollusc has merely the power of 



limiting the places where it is deposited. But the 

 habit, once formed, of being coloured by this com- 

 bination of oxides has been preserved to the present 

 day, and several recent Lueinae, similarly coloured 

 — white outside and yellow inside — give the same 

 result on analysis. As, however, this variety of 

 iron is in many cases not so abundant as it was in 

 the locality where the habit was first acquired, 

 the markings of Lueinae, although formerly very 

 constant, are now extremely variable. Individuals 

 can be found of the same species in which the 

 colour of the interior varies from white to orange. 

 A good example is Lueina striatus (fig. 1, Nos. 10 

 and 12). Two Oligocene fossils, Olivella impressa 

 and Auricula douvilloi, were tinted by the same 

 substance as these Lueinae, but on the outside, as 

 they lived on dark estuarine deposits. 



Pliocene fossils were coloured with the same 

 pigment as recent shells, except that the rapidly 

 diminishing races of British Muricidae and Volu- 

 tidae were still coloured by iron. 



Summing up all the evidence here given, the 

 history of the various pigments seems to be some- 

 thing like this -.—The most ancient one is iron ; all 

 shells were originally either white or coloured by 

 it. As soon as some species began to exist in 

 fresh water or on land, they learnt perforce to 

 produce an organic pigment. The latter was at 

 first nearly insoluble in all species, after a time, 

 before the Eocene epoch ; but subsequent to the 

 branching off of Cgclostoma, and even Helieina, 

 from the main body of land shells, a more soluble 

 colouring matter was produced, capable of being 

 stained blue by nitric acid, and which has con- 

 tinued to the present time. Among marine shells 

 the power of producing insoluble markings was 

 soon acquired by the Trochidae and their allies, 

 most of which at a very early period found it 

 necessary for their protection to have a waved or 

 dotted pattern. Although these shells were smooth, 

 the pigment was prevented from spreading by a 

 mordant. But all the bivalves and the majority 

 of gasteropods continued to be marked by iron 

 down to Eocene times. It is most improbable that 

 a genus which had acquired the power to produce 

 markings caused by organic matter would lose it 

 and again revert to the iron-stained condition. 

 Accordingly, as we find in British formations 

 typical of the Eocene period — Yolutidae, Murieidae, 

 etc.— coloured by iron, we conclude that shells of 

 those genera had never previously possessed any 

 other pigment. Towards the close of the Eocene 

 epoch, owing to various causes, some of which I 

 have described, most species gained the power of 

 forming an organic colouring matter; and since, 

 owing to the large size and diverse forms which 

 many shells had acquired, a mordant was no 

 longer necessary, there was produced the pigment 

 characteristic of most univalves, and the more 

 specialised bivalves, of the present day. 

 Norman Court, Southsea. 



