16 Guide to the Mollusca. 



forests, deserts, marshes, and mountains. A species of the Slug 

 Anadenus occurs at 10,400 feet in the Himalayas, but this record 

 is surpassed by a freshwater Snail which has been found at 18,000 

 feet. The most northerly point at which land Mollusca have been 

 obtained is in 72° N. lat. in South Greenland ( Vitrina angelica). 



A certain number of Gastropod and Lamellibranch genera 

 contain exclusively brackish water species. Certain forms of 

 Paludestrina, Auricula, and Scrobicularia are confined to coastal 

 ditches, estuaries, and creeks, while the estuaries of large tropical 

 rivers contain a special molluscan fauna. The distribution of 

 land forms is in general determined by the amount of moisture 

 and to a less degree by the nature of the soil and possibly by 

 temperature. Individual species may be very rigidly limited 

 in their distribution by obstacles to dispersal. 



The. Mollusca are abundantly represented in the fossil state 

 and are found from the Lower Cambrian upwards. The Gastro- 

 poda are probably the earliest that can be recognized, but the main 

 classes were differentiated at least as early as the Silurian epoch. 



The Mesozoic era was remarkable for the great abundance and 

 variety of the Cephalopoda (Ammonites, Belemnites, &c). The 

 Tertiary fauna as a whole is very like that of the present time, 

 and a great number of existing genera originated within the era. 

 Some are, however, much older, one of the most remarkable being 

 the Gastropod genus Pleurotomaria, which is recognizable in 

 Silurian rocks and has survived until the present day. 



For a more complete account of fossil Mollusca the visitor is 

 recommended to consult the Guide to the Fossil Invertebrate 

 Animals. 



Variation. 



Variation among Mollusca is illustrated by some selected 

 examples exhibited in Wall-case A and Case 135. 



Variation is sometimes directly dependent on changes in the 

 external conditions of life. Thus various Nudibranchs change 

 their colour according to the plants or animals on which they 

 feed, Fiona marina, for example, becoming greyish blue or brown 

 according as it feeds on the Coelenterate Velella or the Barnacle 

 Lepas. 



Changes in the composition of the water may induce marked 

 alteration in the form and shape of the shell in aquatic Mollusca. 

 A decrease in salinity will often bring about a decrease in the 

 thickness of the shell in Patella, Buccinum, and Littorina. In the 

 relatively fresh water of the Baltic Sea the marine Gastropods 

 Nassa reticulata and Littorina rudis are notably shorter than those 

 in the North Sea, while the proportions of the shell in Cardium 

 edule are modified by fresh water in the Sea of Aral. That 

 excess of salinity may have a similar adverse effect upon the 



