xlviii INTRODUCTION. [CH. 



growth of shells is very rapid when it does take place. 

 4th, Most species bury themselves in the ground to 

 increase the dimensions of their shells. Helix pomatia 

 and many other shells retreat for that purpose in summer, 

 having their heads and the mouths of their shells down- 

 wards (this position being reversed during hibernation) ; 

 H. rotundata burrows into decayed wood for the same 

 purpose ; while Pupa umbilicata, Clausilia nigricans (C. 

 rugosa), and Bulimus obscurus bury their heads only while 

 the increase takes place. With regard to marine shells 

 it may be observed that sea-water does not everywhere 

 contain the same relative proportion of mineral and 

 chemical ingredients, and that the admixture of fresh 

 water has a material effect on the substance, texture, and 

 composition of sea-shells. In estuaries, where the water 

 is brackish, oyster-shells are smaller and thinner than 

 usual, owing to the deficiency of calcareous salts ; while 

 the shells of oysters procured from considerable depths 

 in the open sea and at some distance from the land are 

 remarkably large, thick, and heavy. 



Colour. The dye by which the outer layer of shells 

 is stained, and which often forms bands, streaks, spots, 

 and other markings of the most beautiful and varied 

 hues, is secreted by special glands of the mantle. Owing, 

 however, to some defect in this organ of secretion, the 

 colouring-matter is occasionally wanting; and nearly 

 every kind of shell, which is usually tinted, has what is 

 termed an " albino " or white variety. This is the case 

 with regard both to land and marine Testacea. 



The late Professor Forbes, in a paper which was 

 published in the ' Proceedings ' of the Royal Society*, 

 entitled " Note on an indication of depth of Primasval 

 Seas, afforded by the remains of colour in fossil Tes- 



* Vol. vii. p. 21. 



