54 



As has beeu already showu, the Islands of the Canaries, Madeiras, 

 and the Azores possess a marine fauna closely allied to that of the old 

 continent, notwithstanding that the prevaihng set of the currents is from 

 America. Very few mollusca are common to both sides of the Atlantic, 

 except such as are inhabitants of the Arctic Seas, and extend along the 

 coasts radiating from that centre. Out of about 160 species of shells of 

 the Canary Islands and Mediterranean, of which I sent specimens to 

 the late Professor C. B. Adams, he informed me that he could only 

 identify one (Coin mbella crihraria) with a West Indian species — he had 

 probably overlooked Neritina viridis, and perhaps one or two others. 



Saxicava arctica appears to be the most cosmopolitan of mollusks ; 

 belonging, as its specific name imports, to the Arctic Seas, but able to 

 accommodate itself to a variety of cHmate, and to all zones of depth, as 

 far as about a hundred fathoms ; it has been brought from Spitzbergen, 

 China, Behrings Straits, California, and Australia. This, with perhaps 

 a few others, forms an exception to a general law which appears to limit 

 the range of species in animals of this class. 



The distribution of mollusca depending partly on the nature of the 

 sea bottom, and on other conditions diiScult of appreciation, is liable to 

 great intervals of space. Neara costellata, inhabiting the Mediterranean, 

 Canaries, Madeira, and Norway, has only been met with at one point 

 (Loch Fyne) between the first and last-mentioned localities, or in about 

 20 degrees of latitude, and there are other species which present parallel 

 circumstances. 



Before concluding, I may be allowed to make a few remarks on the 

 distribution of Land Mollusca, which must, it is evident, be affected by 

 many conditions, different from those which influence the spreading of 

 their marine relatives. 



We find among snails and allied genera an astonishing variety of 

 habit. Some affect moist situations and dense forests never penetrated 

 by the rays of the sun, feeding voraciously upon a rank vegetation ; while 

 others prefer the most arid tracts, where their food must be scanty and 

 deficient in succulence ; some, like Bulim us decollntus, pass most of their 

 time buried in the earth ; while others, " through winter's cold and- 

 summer's parching heat," select exposed situations, and are enabled to 

 retain their vital powers through extreme changes of temperature. Of 

 these our little Helix umbiUcata and Helix pisana may be quoted as 

 examples. Most species seek shelter in the crevices of rocks, and 

 under stones. 



Although many terrestrial mollusca are capable of enduring not only 

 extremes of heat and cold, but of existing for an astonishing long period 



