CHAP. XX II.] 
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 1IL—OPIBTHO-BRANCH1ATA. 
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. 
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