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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.