526 
GEOGRAPHICAL ZOOLOGY. 
[part iy. 
distribution of these animals is the geological antiquity of the 
group, and the amount of change exhibited in time, by species 
and genera. Now we find that most of the genera of land-shells 
range back to the Eocene period, while those inhabiting fresh 
water are found almost unchanged in the Wealden. In North 
America a species of Pupa and one of Zonites, have been dis- 
covered in the coal measures, along with Labyrinthodonts ; and 
this fact seems to imply, that many more terrestrial molluscs 
would be discovered, if fresh-water deposits, made under favour- 
able conditions, were more frequently met with in the older 
rocks. If then the existing groups of land-molluscs are of such 
vast antiquity, and possess some means, however rarely occurring, 
of crossing seas and oceans, we need not wonder at the wide and 
erratic distribution now presented by so many of the groups ; 
and we must not expect them to conform very closely to those 
regions which limit the range of animals of higher organization 
and less antiquity. 
The total number of species of pulmoniferous mollusca is about 
7,000, according to the estimate of Mr. Woodward, brought down 
to 1868 by Mr. Tate. But this number would be largely in- 
creased if the estimates of specialists were taken. Mr. Woodward 
for example, gives 760 as the number of species in the West 
Indian Islands ; whereas Mr. Thomas Bland, who has made the 
shells of these islands a special study, considers that there w r ere 
1,340 species in 1866. So, the land-shells of the Sandwich 
Islands are given at 267 ; but Mr. Gulick has added 120 species 
of Achatinellidse, bringing the numbers up to nearly 400,— but 
no doubt several of these are so closely related that many con- 
chologists would class them as varieties. The land-shell fauna 
of the Antilles is undoubtedly the most remarkable in the world, 
and it has been made the subject of much interesting discussion 
by Mr. Bland and others. This fauna differs from that of all 
other parts of the globe in the proportions of the operculate to 
the inoperculate shells. The Operculata of the globe are about 
one-seventh, the Inoperculata about six-sevenths of the w r hole ; 
and some general approximation to this proportion (or a much 
smaller one) exists in almost all the continents, islands, and 
