colder climates, but the notices of extra-European species are so scanty that we cannot form
any idea of their numerical preponderance, nor can we, for the same reason, point out, as may
be done in many groups of animals and plants, any particular centres of distribution where
they are more especially congregated. With the exception of a few oceanic wanderers, such
as Glaums and Scyllaa, which have been met with by almost every voyager, nearly all the
Nudibranchs that have been described by naturalists visiting the southern hemisphere belong
to the Doridida; and these show a much greater variety of forms than are to be found in the
same family with us. I t may, therefore, be inferred that the members of this, family are
proportionally more numerous in warm climates than in the more northern and colder
regions; but were we to take the number of species at present known as a ratio of their
preponderance, we should certainly be very wide of the mark, for it cannot be doubted that a
great deal of the apparent deficiency of other genera, in comparison with the Dondida, in
foreign countries, arises from the want of proper examination, and from the little attention paid
by collectors to the less conspicuous forms. It may therefore be expected that when
naturalists visiting foreign countries shall have their attention directed more especially to this
beautiful race of marine animals, they will not only collect those conspicuous species which,
from their size and colour, obtrude themselves upon the eye, but will search out m their
hidden retreats the smaller, and often more interesting because more varied, forms that belong
to the other families. We cannot doubt that a rich harvest is yet in store for future voyagers
in this department of natural history.
To show the futility of founding generalisations upon imperfect data, we may mention
that in a report on the distribution of this order formerly published, we stated on the
authority of existing works, that the Doridida of the Mediterranean greatly exceeded the
EoKdida in number, and that the latter were not only few but of small size, compared with
those of colder climates. The more recent Catalogue of the Molluscs, of the Gulph of Genoa,
by M. Yerany, however, contains a greater number of species belonging to the latter family
than to the former, and we find the average size is in fact greater in the Mediterranean
Bolides than in those of our own shores. But, making every allowance for errors, there still
exists "a greater proportion oiEolidida in the seas of northern Europe, so that our former
inference that Bolls is a northern type of form is probably correct. A comparison of the
British and Mediterranean Nudibranchs shows that not more than five species have been
ascertained to be common to both localities. These are Antiopa cristata, Goniodoris castanea
Polvcera guadrilineata, Idalia elegans, and Boto coronata; others that have been, described
under the same name are certainly distinct, and even those we now mention are not aE quite
free from doubt __ .
The few species that are yet recorded as inhabitants of the seas of North America come
much nearer to the British forms. Two of them, Bendrmotm Begmlddi (arborescent) and
Pohcera illuminata (Lessonii); appear to be identical, and some others are probably so. It is
only on the' Atlantic shores of Europe, however, that we find any considerable number of
species similar to our own. But here again the want of sufficient data prevents our entering
upon any exact analysis. The French coast has been very imperfectly explored, and those of
Spain and Portugal are, in this respect, nearly a complete blank. The species found in the
north of Franoe, on the borders of the British Channel, are, with one or two exceptions,
similar to what are found on the opposite coast of England. Concerning the shores of
Holland and western Germany, our information is very scanty, but as they are generally low
and sandy, the Nudibranchs found on them are probably few. Those of Bomme's species
that have been recognised are identical with our own. As we approach the north of Europe
our information concerning these animals becomes more extensive. At least two thirds of the
species in the Scandinavian fauna agree specifically with those of Britain, and, when both
countries are more completely searched, it is not unlikely that this agreement may be found
yet more intimate.
The distribution of the British species will be best understood by an examination of the
following tables, in which we have endeavoured to condense all the information on the subject
we at present possess.
It has been considered sufficient, in the table of Geographical Distribution, to adopt the
natural and obvious division of the shores of the British Islands into three regions—southern,
eastern, and western—forming the three sides of a triangle whose apex is the north. The
northern forms may by this means be somewhat divided; but this division is naturally
suggested by the form of the coast line, and we the more willingly adopt it, as so little is
known of the littoral Nudibranchs inhabiting the extreme north-western shores. The true
northern marine fauna of our Islands will be found on the north-eastern coast, extending from
Shetland to the Humber; a circumstance arising from the set of the tide from the north in
that direction. Scarcely anything is known of our Mollusca from the Humber to the Straits
of Dover. Properly speaking, therefore, the three known divisions of our shores resolve
themselves into the southern, the western, and the north-eastern. The southern coast is the
richest in specific and generic forms, more especially the latter. Seven genera inhabiting the
southern and western coasts are not found on the eastern: these are Thecacera, Scyllaa,
Lomanotus, Fiona, Alderia, Antiopa, and Proctonotus. Not a single genus of the latter division,
however, is absent in the other two, though Embletonia ought perhaps to be considered a
northern form. As has been observed in other departments of the Mollusca, the southern
species extend much further northward on the western shores of Great Britain than on the
eastern. This arises from the set of the currents on that side of the island in a northerly
direction; and the influence of the Gulph Stream on the western shores of Ireland and
Scotland seems to have a similar effect. On this account probably it is that two or three
species inhabiting our southern and western shores, though not found on the east coast,
make their appearance again on the western shores of Norway.
In the table of distribution according to zones of depth, we have adopted the three great
divisions of littoral, coralline, and deep-water. The laminarian zone, extending from
low-water mark to a few fathoms in depth, we consider to be a sub-region belonging to the
littoral, and characterised mainly by the same species, nearly all the inhabitants of the
Laminarian zone being occasionally found within tide-marks. The two together constitute
the region of sea-weeds or Alga. It would be easy to subdivide this region into smaller
zones, each characterised by one or two particular species, but we have thought it unnecessary
to do so in the present instance. The limits of each species, as far as they have been
ascertained, are mentioned in the specific descriptions. A glance at the table will show that
the greatest number of species is to be found in the littoral zone; though the number