280 



RELICS OF COLORATION IN FOSSIL SHELLS. 



By R. BuLLEN Newton, F.G.S. 



Read Sth March, 1907. 



PLATE XXIV. 



Shell-colobation, as observed among modern Mollusca, usually exists 

 beneath the epidermis, its secretion having been effected by the border 

 of the mantle. According to Edward Forbes,' who was one of the 

 earliest investigators of this phenomenon, such secretion depends 

 largely upon the action of light, so that shells found in shallow waters 

 would be, as a rule, more brilliant than those obtained from considerable 

 depths. His studies of the bathymetrical distribution of existing 

 molluscs enabled him to infer that " well-de6ned patterns were, with 

 very few and slight exceptions, presented only by testacea inhabiting 

 the littoral, circum-littoral, and median zones. In the Mediterranean 

 only one in eighteen of the shells taken from below 100 fathoms 

 exhibited any markings of colour, and even the few that did so were 

 questionable inhabitants of those depths. Between 35 and 55 fathoms 

 the proportion of marked to plain shells was rather less than one in 

 three, and between the sea-margin and 2 fathoms the striped or mottled 

 species exceeded one half of the total number." He then surmised 

 that, as vivid patterns are not presented by testacea living below 

 certain depths, it might be possible to indicate, within certain limits, 

 the depth of Palaeozoic seas from a study of their molluscan fauna 

 exhibiting traces of colour. Forbes further stated that "original 

 colour is very rarely exhibited by fossil shells ; occasionally we meet 

 with specimens in which, owing probably to organic differences in the 

 minute structure of the coloured and colourless portions of the shell, 

 the pattern of the original painting is clearly distinguished from the 

 ground tint." He also thought that as Mesozoic and Tertiary shells 

 are closely related to existing types, there should be little difficulty in 

 ascertaining the probable bathymetrical zone of the sea in which 

 they lived. With regard to Palaeozoic strata such calculations he 

 considered more difficult, because the "general assemblage of articulate, 

 molluscan, and radiate forms is so different from any now existing 

 with which we can compare it, and so few species of generic types 

 still remaining are presented for our guidance, that in many instances 

 we can scarcely venture to infer with safety the original bathymetrical 

 zone of a deposit from its fossil contents." However, after referring 



' " Report on the Mollusca aud Radiata of the jEgean Sea, aud on their Distribution, 

 considered as bearing on Geology" : Rep. Brit. Assoc, 1843-4, pp. 172, 173. 

 "Note on an Indication of Depth of Priniffival Seas, afPorded by the Remains of 

 Colour in Fossil Testacea": Proc. Roy. Soc. Lond., vol. vii (1854), pp. 21-23. 



