DISPERSAL OF SNAILS 
269 
We are too apt to argue that the fauna of an island 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 Tomocyclus. The centre 
of dispersal is Cuba, chiefly the western part of the island. 
Prom 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 great 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 pf 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 
pccasion to state, are exceedingly difficult to discriminate 
