526 GEOGRAPHICAL ZOOLOGY. [part iv. 



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 Piipa 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 tliat there were 

 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 whole ; 

 and some general approximation to this proportion (or a much 

 smaller one) exists in almost all the continents, islands, and 



