America the discrepancy is less, but the records from both sides are too meagre for
positive conclusions.
The pelagic forms such as Phylliroe, Scyllaea, Glaucus, and Fiona, appear to be
cosmopolitan and differ little in the various seas ■which they frequent. Perhaps the same
is true of deep-sea forms, but the data are not sufficient to support any conclusions.
Different species of the genus Bathydoris have been found in the Arctic Atlantic (1870
fathoms), the Equatorial Pacific (2425 fathoms), and in the Antarctic at moderate
depths.
I I I .
NOMENCLATURE.
T he nomenclature of the Nudibranchiata has been considerably modified since the time
of Alder and Hancock, and changes took place even during the publication of the Monograph,
for the names used in the Synopsis of G-enera (Part VII, Appendix, pp. xvi—xxiv)
are in many cases not those used in the body of the work. Such changes are due to two
principal causes: Firstly the discovery that there are objections to the use of a name
(the commonest objection being that it is already employed for some other animal), and
secondly the fusion or subdivision of existing species and genera.
Alder and Hancock themselves recognized that Eumenis and Oithona must be
discarded as preoccupied. Antiopa, Diphyllidia, and Gavolina are in the same plight, and
the specific names of Glaucus forsteri and Fiona nobilis have been replaced by atlanticus
and marina. I do not, however, think it is proved that Gadlina repanda is synonymous
with the Doris obvelata of 0. F. Müller, as some authors contend, and unless the identity
is beyond all dispute it seems a pity to make any change. I confess that I sympathize
with those zoologists who wish to be as conservative as possible in applying the rule
of priority. I t is a mere means for conveniently deciding disputed cases, not a moral
law which must be enforced whether convenient or not. When two names are both
in practical use it is most desirable to eliminate one. Otherwise there is confusion, and
if, as sometimes happens, both names are retained in lists, the genus may be credited with
an extent and an abundance of species which are deceptive.1 But I cannot see that
anything is gained, whereas much is obviously lost, by discarding a wellTknown name
in favour of an obscure and forgotten one when there is practically no competition
between the two. For instance Alder and Hancock originally described an animal
as Eolis pallida. Then in the Monograph they altered the name to Eolis tricolor, because
they found that the colours of the animal were not habitually pale. According to a rule
subsequently accepted by zoologists this alteration was incorrect, and the rule is a good
one for the guidance of authors, since changes in names once given are confusing. But
nothing whatever is gained by the learning which disinters the forgotten designation
pallida and substitutes it for the ti'icolor of the Monograph, and much inconvenience is
caused, for the species has been described and figured by Bergh and Trinchese under
the name of tricolor, so that in this and very many other cases the best information about
an animal would not be found under its authorized name. A specialist may thread his
way through the labyrinth of synonymy, but a naturalist consulting the literature of a
group with which he is not specially acquainted is baffled at every turn by new names
which correspond to little in the descriptive literature. Equally unnecessary and far
more inconvenient is the substitution of Tethys for Aplysia as the name of a well-known
Tectibranch, and the consequent use of some other designation (e. g. Phenicurus) for the
1 The genera Doto, Platydoris, Discodoris, Ma/rionia, and many others are in this case.