not too rigid) there is no harm in retaining the specific names if the different varieties or
races are found in different districts, as is often the case. But if eggs laid by similar
parents in the same locality produce indifferently D. vemicosa and D. maculata, then the
two forms must be registered as mere varieties. But though we are apt to overlook the
luxuriant variability of the Nudibranchiata if we pay attention only to the series of forms
selected as specific types, yet it must be admitted that, taking the classification as a
whole, species have been unduly multiplied, and that of those registered at least a third
are superfluous.
The question of variation and specific distinction is intimately connected with
another, namely the distribution of Nudibranchs. I t is unfortunately impossible to
treat this interesting subject with the accuracy one would desire, because though the
Nudibranch faunas of the north-eastern Atlantic and the western Mediterranean are
well known, the available data for the tropical Atlantic are extremely scanty, and the
specific identity of northern and southern forms is often uncertain.
The British coast-line extends over about thirteen degrees of latitude, and it is not
surprising to find that two faunistic districts overlap within this considerable area. As
Alder and Hancock have pointed out, the northern fauna is continued far south on our east
coasts, whereas on the western side the northerly set of the currents and the influence of
the Gulf Stream carries southern forms to the western coasts of Scotland and Ireland.
Thus Dd'is verrucosa, which according to all other records is a definitely southern form,
has been found, not only near Plymouth in England, but at Ballynakill and in the Firth
of Clyde. The meaning of this phenomenon seems to be not so much that the northern
fauna fails to reach our southern coast, as that on the western coast it is reinforced by a
certain number of southern genera and species;—But some northern forms, such as
Aldisa zetlandica, Cadlina repanda, Goryphella salmonacea, have not been recorded from
our southern coasts.
The fauna of Scandinavia is substantially the same as that of the northern parts of
England and Scotland, and a considerable number of its species penetrate to the Arctic
regions. The following forms and perhaps others have been taken within the Arctic
Circle:—
Dendronotus, 3 species.
Doto coronata.
Pleuroleura walten.
Gratena or Guthona, 3 species.
Goryphella, 7 species.
Gumanotus laticeps.
Favorinus albus.
Chlamytta, 4 species.
Hero formosa.
Flysia viridis.
Limapontia nigra.
Doris tubei'culata.
Aldisa zetlandica.
Cadlina repanda.
Acanthodd'is, 2 species.
Adalaria proximo.
Lamellidoris, 2 species.
Polycera, 2 species.
Issa, 2 species.
Tnopa lacera.
Idalia pulchella.
Ancula ciistata.
To these should doubtless be added other Nudibranchs which have been captured
in northern but not strictly arctic waters, such as Bathydoris and Doridoxa. But the list,
taken as it is, probably gives a fair idea of the arctic fauna. ‘ The only genera in it which
are unknown in the temperate Atlantic are Ghlamylla and Pleuroleura.1 The Clado-
hepatica number twenty-four and the Holohepatica fifteen species. Among the former
iEolids (sixteen) are predominant: among the latter phanerobranchiate Dorids (twelve).
The true cryptobranchiate Dorids are only three.
The fauna of the Atlantic coast of France appears to be practically the same as that
of south England. All the Nudibranchs recorded by Hecht from Roscoff near Brest are
described by Alder and Hancock. At Arcachon,3 too, the majority of Nudibranchs
recorded are also known at Plymouth, but southern forms such as Spurilla mediterranea
begin to make their appearance, and the commonest Dorid is D. verrucosa, which is rare
with us. But it is not till we reach the coast of Portugal that the southern or subtropical
element contributes a considerable proportion of names.
The Nudibranchs of this region, as described by M. d* Oliveira,3 comprise the
following:
Doridopsis and 1
Dvriopsttla.,
Tritonia, 1.
Marionia, 1.
Dotis, 2. Facelina, 4.
Jorunna, 1. Goryphella, 1.
Platydoris argo. Amphoi’ina, 1.
Gliromodons, 3. Molidiella, 1.
Goniodoris, 1. Doto, 1.
Polycera, 1. Herm-aea, 1.
Triopa, 1.
Elysia, 1.
Pleurophyllidia, 2.
In the above list the genera Doridopsis, Doriopsilla, Ghromodoris, Platydoris,* and
Marionia are characteristic of the warmer seas, and Portugal is probably their northern
limit in the Atlantic.
The Nudibranchiata of the eastern coast of the United States and Canada—that is,
of the north-west Atlantic—are not well known, though they, have formed the subject
of several publications.5 The lists which have been compiled are probably not
exhaustive, and it is difficult to say how far the forms described are specifically the same
as those found in the north-eastern Atlantic or merely similar to them. In any case the
resemblance between the two faunas is close. The principal American genera are :
Goryphella, 9 sp. Embletonia, 2.
AEolidia papillosa. Fiona, 1.
Gratena (Guthona), 4. Scyllasa, 1 or 2.
Galvina, 2. Dendronotus, 2 or 3.
Tergipes, 1. Doto, 2.
1 The distribution of this genus is very strange. All the other known species inhabit the
tropical In do-Pacific.
3 Cuenot, Doridiens d’Arcaclion, 1901; id. Eolidiens d’Arcachon, 1906.
3 Opistliobranches du Portugal, Coimbra, 1895.
4 N.B.—The designations Platydoris testudinaria and P. planata are not correct.
5 Gould, Invertebrata of Massachusetts, and several catalogues <by Verrill.