DISPEESAL OF SNAILS 269 



We are too apt to argue that the fauna of an isla^nd covered 

 by recent volcanic deposits must necessarily have been derived 

 by occasional means of dispersal. I need only cite the case of 

 the Galapagos islands, which are entirely volcanic, and have 

 seemingly risen from the floor of the ocean. Nevertheless, 

 it can be demonstrated from a faunistic point of view, as I 

 shall show later on, that they have once formed part of an 

 ancient continuous land surface. 



Still another group of operculate land snails contains the 

 two genera Megalomastoma and Tomocyolus. The centre 

 of dispersal is Cuba, chiefly the western part of the island. 

 From there Megalomastoma reached Haiti, Portorico and the 

 Virgin islands, which lie close to the latter. Tomocyclus 

 inhabits only southern Mexico and Guatemala. Thus it 

 seems probable that Cuba, Guatemala, and southern Mexico 

 were connected with one another by land in very remote 

 times. 



Let us take as another example, that of the ancient and 

 large family Bulimulidae. In another chapter I have dwelt 

 on the gteat age of this family of snails (p. 209) and its 

 general range. It contains mostly large, ponderous snails 

 with somewhat conical shells. Anatomically they are re- 

 lated to the Helicidae. Although represented by a great 

 many species, only a few genera enter the West Indies. One 

 of these (Plekocheilus) inhabits almost exclusively Guiana, 

 Venezuela, Colombia, Ecuador, Peru and Bolivia, that is to 

 say the northern and western States of South America, where 

 the different species are often found at great heights in the 

 mountains. Only two species enter the West Indies, viz., 

 P. aurissileni, which is peculiar to St. Vincent, and P. aula- 

 costylus, which is only met with in the island of St. Lucia. 

 These islands are two pi the most southern group of the 

 Lesser Antilles. The ancestors of the two species of Pleko- 

 cheilus have probably entered these islands when the latter 

 were connected with one another and with the mainland of 

 Venezuela, and, as the species are very distinct from one 

 another, this could not have happened within very recent 

 geological times. 



The species of the genus Bulimulus, as I have already had 

 occasion to state, are exceedingly difficult to discriminate 



