THE STORY OF THE LAND SNAILS 345 



shell grows rapidly, the first growth being thin and 

 transparent. At the close of the rains the Oxy- 

 stylas form a dark border around the mouth of the 

 shell, but the Liguus rarely and then to a much 

 less extent. The first season's growth may con- 

 sist of from four to six turns or whorls, the second 

 of perhaps a little less than one whorl, and after 

 that the growths are short. By counting these 

 rest marks it is possible to guess at the age of the 

 snail, which under favorable circumstances prob- 

 ably lives four or five years. 



All our Liguus and Oxystylas are derived from 

 the American tropics, the former from Cuba, and 

 the latter, no doubt, from a species of rather wide 

 Antillean and tropical American distribution. In 

 another chapter I have given reasons for believing 

 that there has been no land passage between Cuba 

 and Florida since the present life has existed. So 

 far as we know the animals and eggs of these tree 

 snails sink in salt water, and it is hardly possible 

 that birds or hurricanes could have transported 

 them. But in some way they must have made a 

 considerable sea voyage, and the manner in which 

 they have accomplished this is of great interest. 

 These and other tropical snails must have been 



