1896.] NATURAL SCIENCES OF PHILADELPHIA. 395 



INSULAR LANDSHELL FAUNAS, ESPECIALLY AS ILLUSTRATED BY THE 

 DATA OBTAINED BY DR. G. BAUR IN THE GALAPAGOS ISLANDS. 



BY WILLIAM HEALEY DALL. 



INTRODUCTORY. 



The Galapagos Islands, lying under the equator about 90° west 

 of Greenwich, comprise two principal groups separated by nearly 

 1,200 fathoms of water. One of these groups, northwest of the other, 

 contains only Culpepper (550 ft.) and Wenraan (830 ft. elevation) 

 Islands and a few insignificant rocks. Culpepper, owing to its 

 small elevation, is nearly barren, while Wenman shows on its upper 

 surface a thin coating of grass and other vegetation. From neither 

 of these has any collection been made or is any land shell known. 



The main group of the Galapagos rests on an elevation of the sea 

 bottom included within the 1,000 fathom line. It may be provision- 

 ally divided into three groups, a southeastern, a central and a north- 

 eastern, in all about a dozen islands and some smaller islets and 

 rocks. 



The southeastern group comprises Charles, Chatham, Hood and 

 Barrington Islands. Hood is destitute of water in the dry season 

 and green only in the wet season, owing to its small elevation which 

 does not bring it into the region of condensing clouds. Much of 

 the surface is covered with blocks of lava. Chatham and Charles 

 are among the most fertile islands of the group. 



The central islands include the largest of the whole, Albemarle, 

 which appears to consist of several primitive islands united by low 

 areas of volcanic material ; Narborough, which exhibited volcanic 

 activity as lately as 1836 ; James ; Indefatigable, and the much 

 smaller Duncan Island, besides a number of islets. 



The northeastern group comprises three comparatively small 

 islands Abingdon, Biudloe and Tower. 



The floral characteristics of the Galapagos have been mentioned 

 by Darwin, fully discussed by Hooker and well described by Wolf, 

 while Tanner, Baur and Agassiz have added the facts gathered by 

 later explorations. I shall, therefore, merely briefly suoimarize the 

 characteristics which these writers have noted. 



