MASON : VARIATION IN THE SHELLS OF THE MOLLUSCA. 339 



brilliantly coloured the more sunshine they get, and the species 

 from the deeper waters of the sea are usually white or light 

 pink in colour. This holds true, not only of the whole shell 

 but also of the different parts of the individuals themselves, 

 that part most exposed to the light showing the brightest 

 colouration ; and in those species permanently fixed, the side 

 protected from the light shows none of the patterns which 

 may exist on the exposed surfaces, and the same shell, like 

 some of the Patellce and Crepidulce^ may show a difference when 

 they have changed their habitat to a place where they received 

 a more to a less free exposure to light, the markings showing 

 much more either on the younger or older portion of the shell. 

 The most brilliant of all the sea shells are those inhabiting the 

 littoral zone, where we get red, orange, yellow, blue, brown, 

 black, and even white, of much greater purity than in those 

 from greater depths. 



In a similar manner, as we near the equator, the colours 

 increase in brilliancy \ no one can fail to observe the difference 

 in a number of shells collected from the Northern or from the 

 Tropical Regions. The same thing holds good with land shells. 

 Even in our own seas I have noticed that the colours of the 

 littoral specimens gathered from the shores of Cornwall — in 

 places like Kynance Cove and Newquay— are remarkably 

 brilliant ; here there is no mud to foul the water and obstruct 

 the light, the sands themselves being remarkable for their 

 cleanliness and bright colours. 



In some cases it is possible that certain colour variations 

 may have influence in protecting them from being preyed upon, 

 and so tend to become predominant in the species. 



All moUusca, the shells of which are adorned by patterns, 

 vary much in the disposition of the pigment-producing glands. 

 Thus, Helix ericetomni varies from a uniform white, through 

 pink, to a deep brown colour, entirely without markings, while 

 it may have five bands running round each whorl, these bands 

 may coalesce in every possible way, any one may be absent. 



