424 WEST INDIAN MOLLUSKS—SIMPSON. 



Between Cuba and Jamaica there is a great trough some 3,000 fath- 

 oms in depth, known as the Bartlett Deep. Biunning nearly due west 

 from Cape Cruz on the south side of Cuba, and north of the profound 

 abyss is a shoal on which are the Cayman Islands, but which deepens 

 to 1,500 fathoms before reaching Belize. To the southwest of Jamaica 

 a wide shoal extends to the coast of Honduras, forming the Mosquito, 

 Baxonuevo, Savanilla, Rosalind, and Pedro banks, along which are scat- 

 tered low islets, and which, with an elevation of 500 fathoms, would 

 connect Jamaica with the continent. The western end of Cuba points 

 directly toward Cape Catoche, fi'om which it is separated by a strait 

 130 miles wide and 1,104 fathoms deep. The 100-fathom line would 

 unite Cuba to the Bahamas. At the southeast the Greater Antilles are 

 separated from the Lesser by the Anegada Channel, which carries iu a 

 depth of 1,100 fathoms from the Atlantic, ending in a deep basin between 

 Santa Cruz and St. Thomas of 2,400 fathoms. East of this a ridge 

 crosses it which comes within 000 fathoms of the surface. 



The Lesser Antilles have not a central nucleus of igneous or nieta- 

 morphic rock. In referring to this subject Alexander Agassiz says:* 



The position of tlie most recent Pliocene and Post Pliocene beds seems to indicate 

 that some of the volcauoes now active in the West Indies date hack to the Pliocene 

 period, and others to the Post Pliocene. The islands to the north of Guadeloupe form 

 two parallel chains, the western consisting of Saba, St. Eustatius, St. Kitts, Nevis, 

 Redouda, and Montserrat, all of which are volcanoes of Post Pliocene date ; while to 

 the eastward is a chain of volcanoes of Tertiary age— Sombrero, Anguilla, St. Martin, 

 St. Bartholomew, Barbuda, and Antigua. At Guadeloupe the recent islands are 

 directly united with the volcanic chain, and the still more recent limestones are 

 found on its western shores. 



Agassiz and other authorities agree that the northern portion of the 

 Lesser Antilles is of much more recent origin than the greater archi- 

 pelago, and the volcanic chain no doubt rests upon a submarine plateau. 



The Bahama group is also believed to be of somewhat modern origin. 

 Agassiz thinks that it was formed on an extensive shoal, from the 

 remains of marine animals deposited at a time when the current from 

 the Gulf of Mexico flowed over the area of the present archipelago with 

 very much less velocity than it does at present.! All the islands are 

 low, and many of them are of coral formation. 



Before entering into details as to the distribution of genera and 

 species it may be well to say a few words as to the means of'dispersal 

 of the land and fresh-water mollusks. Where closely related forms 

 or groups are found in lands sei)arated by the sea, I think we may 

 generally conclude that they have reached their present distribution by 

 one or more of the following means: 



First: — By former land connection which hasexisted within the life- 

 time of present species or groups. 



Second: — By the sea, by means of oceanic currents, winds, or storms. 

 It is very well understood now that many land, and some fresh-water 



* Three cruises of the Blake^ i, p. 109« 



tojj. cit., p. 75. 



