LAND SHELLS : THEIR TENACITY OF LIFE. 93 



to the islands than the nearest now existing countries. 

 The Azores or Western Islands^ lying at a great distance 

 from the nearest continental land, and separated there- 

 from by an ocean of great depth, have been specially- 

 referred to by Mr. Wallace as typical " oceanic islands," 

 and they possess land-shells in some plenty, sixty-nine 

 species having been recorded for the group, thirty- seven 

 of which are common either to Europe or the other At- 

 lantic islands, and thirty-two peculiar, but almost all allied 

 to European types ; on the other hand, though there are 

 streams, springs, and lakes, apparently presenting the 

 most favourable conditions for the existence of molluscs, 

 not a single fresh-water species has yet been discovered.' 

 The wholly volcanic and strictly oceanic island of St. 

 Helena, only about ten miles long by eight wide, and 

 so remarkable for its extreme isolation, situate as it is 

 nearly in the middle of the South Atlantic, iioo miles 

 from Africa and 1800 from America, possessed twenty 

 truly indigenous species of land-shells (a large propor- 

 tion of which have unfortunately become extinct since 

 the destruction of the forests) ; and several others, pro- 

 bably recently introduced by man, now occur in the 

 island. Here, as in the Azores, fresh-water shells are 

 altogether absent, the streams and tanks of the interior, 

 the trickling rocks, waterfalls, and pools, being all un- 

 inhabited by molluscs of aquatic genera.^ The Sand- 



^ " Island Life, " pp. 236-7, 239-40, 247, and 293 ; ed. 2, pp. 245, 

 247-8, 256 and 305 ; H. B. Tristram, as quoted by Wollaston^ 

 "Testacea Atlantica," 1878, p. 6. 



' " Island Life," pp. 281 and 293 ; ed. 2, pp. 292 and 304 ; WoUas- 



