jy 4 INTRODUCTION". 
to the West Indian islands, in which other peculiar forms 
of the genus prevail. Grlandina is an American genus, 
and confined to the limits which have been already men- 
tioned. Helicina is also an American genus, though 
not exclusively so, the central focus of which is the 
Antilles, whence it is diffused through the other West 
Indian Islands to the adjacent ■ parts of the continent on 
the sea-coast, as far as 25° north latitude. In the 
Philippine Islands this genus re-appears under geo- 
graphical conditions s imil ar to those which distinguish 
the American localities ; that is to say, the position of 
this group of islands and its relations to the neighboring 
continents of Asia and Australia, bear an extraordi- 
nary resemblance to those of the Antilles in respect of 
the continent of America. On all the principal groups 
of islands throughout the Pacific, this genus is found, 
though very essentially modified in form from the 
American types. In conclusion, the genus Cyclos- 
toma seems to range around the whole circumfer- 
ence of the globe within 20° both north and south of the 
equator, avoiding for the most part continental stations, 
and finding the conditions most favorable for its exist- 
ence in the innumerable islands with which this belt of 
the globe is studded. It is diffused more numerously 
in the eastern than in the western hemisphere, in about 
the same proportion as the islands themselves are more 
numerous there. A single species being often confined 
to a small group of islands, or even to a single island, 
and the species in general being very much separated. 
