CHAP, xxii.j MOLLUSCA. 529 



characterised. He also arrives at a series of sub-divisions, 

 which generally (though not exactly) agree with the sub-regions 

 which I have here adopted. The Palsearctic, the Ethiopian, and 

 the Oriental regions, are also generally admitted to be well 

 characterised by their terrestrial molluscs. There only remain 

 the Australian and the Neotropical regions, in which some want 

 of homogeneity is apparent, owing to the vast development and 

 specialisation of certain groups in the islands which belong to 

 these regions. The Antilles, on the one hand, and the Polyne- 

 sian Islands, on the other, are so rich in land- shells and 

 possess so many peculiar forms, that, judged by these alone, 

 they must form primary instead of secondary divisions. We 

 have, however, already pointed out the inconvenience of any 

 such partial systems of zoological geography, and the causes 

 have been sufficiently indicated which have, in the case of 

 land-shells as of insects, produced certain special features of 

 distribution. 



We therefore venture to hope, that conchologists will give us 

 the advantage of their more full and accurate knowledge both of 

 the classification and distribution of this interesting group of 

 animals, not to map out new sets of regions for themselves, but 

 to show what kind of barriers have been most efficient in 

 limiting the range of species, and how their distribution is 

 actually effected, so as to be able to explain whatever dis- 

 crepancies exist between the actual distribution of land-shells 

 and that of the higher animals. 



Order III.—OPISTHO-BBANCHIATA. 



There are ten families in this order, all of which, as far as 

 known, are widely or universally distributed. Some of them 

 are found fossil, ranging back to the Carboniferous epoch. They 

 are commonly termed Sea-slugs, and have either a thin small 

 shell or none. We shall therefore simply enumerate the families, 

 with the number of genera and species as given by Mr. Wood- 

 ward. 



