SHELL-COLORATION IN BRITISH EXTRA-MARINE MOLLUSCA. 63 



calcium carbonate, or other salt of which it might be composed. 

 But "original ancestors" may mean something a good deal more 

 recent than this hypothetical animal, all traces of which in any 

 definite form have long since disappeared. The meaning which 

 may reasonably be given to it is those chronologically most 

 distant molluscans that had organisations from which the various 

 Gastropodous or Lamellibranchiate (for to these two groups, for 

 obvious reasons, the question is confined) forms at present in 

 existence have more or less immediately been derived. 



There is now, I assume, no doubt that the extra-marine 

 mollusca have been derived from marine forms by gradual adapta- 

 tion to terrestrial or freshwater life. The freshwater forms have 

 passed through an estuarine, brackish-water life, while the land 

 species have been derived (1) from sea forms migrating direct 

 across the littoral, (2) from those which have already taken to a 

 freshwater or estuarine habit.* 



The shells of marine Gastropoda are on the whole far more 

 brightly coloured than freshwater forms ; and at the same time 

 are much thicker in substance. This latter enables them to 

 withstand better the greater battering about to which they are 

 subjected by the rough water in which they live, and, assisted by 

 the coloration, to escape their enemies more easily. One would 

 expect to find that marine mollusca would well illustrate cryptic 

 coloration, for their enemies are very numerous and they are 

 practically dead if once seen. As C. A. Westerlundf quaintly 

 remarks, they are not remarkable for celerity of movement, and 

 hence the chief thing in catching them is to find them. But, as 

 a fact, very few cases of protective coloration have been recorded. 

 One or two are given by Prof. Poulton+ where corals are imitated; 

 and a figure of a striped Purpura lapillus taken on striped rocks 

 by Mr. A. H. Cooke. § If, however, we could see with fishes' 

 eyes, we should probably find here, as in other cases, that pro- 

 tective colours are far commoner than we suppose. 



* Further, certain land forms may take to an aquatic life. Limncsa 

 truncatula, e. g. (really more a terrestrial than an aquatic species) looks as if 

 it were migrating one way or the other ; either following allied forms into 

 the water or leading ihem on to land. 



f ' Fundamenta Malacologica,' 1892, p. 17. 



| ' Colours of Animals,' 1890, pp. 70—71. 



§ ' Cambridge Nat. Hist. vol. hi. p. 90, fig. 35 (2). 



