174 INTRODUCTION. 



to the West Indian islands, in which other pecuhar forms 

 of the genus prevail. Grlandma is an American genus, 

 and confined to the limits which have been already men- 

 tioned. Helkina is also an American genus, though 

 not exclusively so, the central focus of which is the 

 Antilles, whence it is diflfused 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 similar 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, avoiduig for the most part continental stations, 

 and finding the conditions most favorable for its exists 

 ence in the innumerable islands with which this belt of 

 the globe is studded. It is difiused 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. 



