Lophocercidse (especially the former) approximate to the Nudibranchs, but also the
Gymnosomata as well as Acclesia and Aplustrum. These two genera each unite several
features which are either common to all the Nudibranchiata or characteristic of some of
them. Thus Acclesia has (1) no shell, (2) a concentrated nervous system, (3) a radula
roughly resembling that of Cadlina, (4) mandibular plates roughly resembling those of
Hexabranchus or Ghromodoris, (5) stomach-plates, (6) an armature of the genitalia.
Aplustrum has (1) an internal vas deferens, (2) teeth rather like those. of Kalinga, (3)
mandibular plates like the labial armature of many Dorids, (4) a blood-gland, (5) pinnate
rhinophorial organs, (6) a concentrated nervous system.1 These genera cannot be said
to be on the border-line between Tectibranchs and Nudibranchs, but if we let our fancy
play with extinct forms (an easy and not very profitable speculation), a short and
plausible series might be imagined leading from either of them to true Nudibranchs.
Land slugs are known to be polyphyletic, and there is no.objection to admitting the same
origin for our group except that certain forms bring divergent subdivisions (such as
Clado- and Holohepatica, Ascoglossa and iEolids) so near together that the hypothesis
may be superfluous. Those who have kept count of the new types discovered in the last
fifteen years will not be surprised if more annectent genera are discovered. But if a
polyphyletic origin is required to explain the facts, I should be disposed to imagine not
that such dissimilar creatures as the Lophocercidse and Phyllobranchus are united by direct
descent, but that there was once a whole series of forms roughly analogous to Tritonia but
possessing characters which at present survive only in relatively primitive Tectibranchs
and highly specialized Nudibranchs. Nor do I think we have any right to assume that
it is only the more specialized Tectibranchs which have produced even more specialized
descendants in the Nudibranchs. Even Acteons the very root of the opisthobranch tree as
it is commonly drawn, is not unlike the Nudibranchs in its buccal organs and genitalia.
The affinities of the Oncidiidge to the Tecti- and Nudibranchiata are also worth notice.
They must be regarded as Pulmonates for they have the pulmonary cavity characteristic
of this group, with which they also agree on the whole in the structure of the genitalia
and kidney. But they also present the following features: (1) The shell is entirely
absent. (2) Besides pulmonary respiration on land they are also capable of aquatic
respiration in the water. This is performed by the integuments which in some species of
Oncidium bear dorsal tufts resembling the branchiae of Dorids. (3) They are opisthobranch,
the gill lying behind the heart. (4) The liver is in three portions, opening into
the stomach by separate ducts. (5) The stomach consists of three or four divisions, the
first with thick walls and chitinous plates. (6) The verge is armed with numerous small
spines. (7) The nervous system is concentrated round the oesophagus and consists of five
ganglionic masses, the cerebro-pleural (fused) and pedal pairs and an unpaired visceral
ganglion. These resemblances have been explained as convergences, on the supposition
that the Oncidiid® are specialized Pulmonates which have taken to marine life, but it is
hard to see how this could affect the structure of the liver or genitalia. Plate is therefore
probably right in regarding them as a very early branch of the Pulmonates, thrown off at
a time when it was possible to combine pulmonate characters with those of the Opistho-
branchiata.
1 Known best in A. albo-ci/nct/tim.
DESCRIPTIONS OF THE SPECIES.
The species which form the subject of the following notices fall into three classes.
First those figured in the drawings preserved at Newcastle. These are sixteen in
number, namely Doris testudinaria, Aldisa zetlandica, Adalaria loveni, Lamellidoris
ulidiana, Grimora papillata, Lomanotus genei, Hero formosa, *Janolus hyalinus, Doto
cuspidata, Embletonia pallida, Amphorina ccerulea., *Guthona (?) northumbrica, Guthona (?)
inornata, *Alderia modesta, Elysia virid/is, Limapontia depressa. The three species
marked with an asterisk are already figured in the Monograph but the drawings now
reproduced give further details. The second class comprises eleven species which are
not described in the Monograph. They are Doris verrucosa, Dons maculata, Pleurophyllidia
loveni, Gumanotus beaumonti, Genia cochsi, Hancoclda eudactylota, Doto cinerea, Goryphella
salmonacea, Stiliger bellulus, Acteonia corrugata, Limapontia nigra. Four of them are
illustrated by figures drawn from living animals in the last two years, chiefly at
Plymouth. Most, of these forms have been added to the British fauna since the time
of Alder and Hancock, but a few of them are mentioned in their later works. The
third class consists of seven species mentioned in the Monograph about which additional
information is now furnished. These are Tritonia alba, G'eitodons planata, Dendronotus
lacteus, Doto pinnatifida (three varieties), Eolis angulata, Galma glaucoides, Aporodoods
millegrana.
TRITONIA ALBA A. & H.
(Plate I, fig. 10.)
See Monograph, part vii, p. 48, and Appendix, p. vij and Eliot L, p. 335.
Specimens of this form were found by Alder and Hancock at Cullercoats, near
Newcastle, and described by them as having considerable external resemblance to young
individuals of T. hombergii, but as differing in dentition from all known Tritonias, inasmuch
as the lateral teeth were denticulate or branched. Bergh (40, pp. 734 and 736)
rejects the species as doubtful, and thinks that the denticles were merely an illusion of the
microscope. An examination of the original specimens preserved in the Hancock Museum
at Newcastle-on-Tyne has shown me, however, that this is not the case, and that the
teeth are really denticulate.
The two specimens are respectively 7 mm. and 6 mm. long, and 2‘8 mm. and 2*2 mm.
broad. One is dark brown, the other yellowish. The hard buccal parts are fortunately