Genus 17. ALDERIA,* Allman.
Corpus ovato-oblongum, sub-convexum, non palliatum. Caput terminate, distinctum, utrinque in
lobum lateralum productum. Tentacula nulla. Maxilla: nulke. Bronchus papillosse, utroque lateri
dorsi dispositse. Anus posticus, in linefc media dorsi situs. Orificium genitale anticum ad latus
dextrum.
This genus was first described by Professor Allman, in a paper read at the York meeting
of the British Association in 1844; and was afterwards published, with a more detailed
description, by the same excellent naturalist, in the ‘Annals of Natural History’ for January,
1846. The single- species on which it is founded had been previously observed by Professor
Loven of Stockholm, who had placed it in the Stiliger of Ehrenberg. M. Loven has since
adopted the genus Alderia in his ‘ Index Molluscorum Scandinaviae.’ This genus has obvious
relationship with Hermcea and Stiliger, from both of which it differs, as well as from the other
genera of the order, in the total absence of tentacles. It is also distinguished from the two
genera above named, by the more expanded form of the foot, as well as by the position of the
anus, which is not placed forward on the back, as in these genera, but occupies a posterior
position, similar to that of Prodonotus and Antiopa. In the absence of tentacles, and in the
broad reflected sides of the foot, it shows an approach to the Bulla tribe, and likewise to a
small Inferobranchiate Mollusk, described by us in the Eighteenth Volume of the ‘Annals of
Natural History,’ and referred, perhaps erroneously, to Pelta of Q-uatrefages. It appears to
us to be also, in some measure, related to Limapontia, which is not only deficient in tentacles,
but has the buccal apparatus and tongue similar to those of Alderia; and the
curious crystalline, spur-like stiletto, in connexion with the reproductive organs, is found in
both. Professor Lov6n remarks of this animal, that it has the branchiae of Holts, the vent of
Doris, and the head and foot of Akera.
The absence of corneous jaws, as well as the circumstance of its having been found on
Ulvae, leave little doubt that Alderia is a vegetable feeder, and this view is confirmed by its
gregarious habits. The genus is more nearly amphibious than any other of the tribe,
living in salt-marshes or estuaries with a muddy bottom, and only partially overflowed by the
tide.
The body is ovate oblong, rather broad, and not much depressed. The head is terminal
and distinct, narrower than the body, and produced into short lobes at the sides. The
mouth is without jaws, and has a narrow spinous tongue. There are no tentacles. The eyes
are distinct, and placed rather laterally on the narrow posterior part of the head. The
branchiae, which are papillose or styliform, are arranged in transverse rows on the sides of
the back : there are few in front, but they become larger and more numerous behind. The
anus is nipple-shaped, and is placed posteriorly on the median line of the back. The foot is
* Named after one of the authors of this work.