MONOGRAPH OF BRITISH LAND AND FRESHWATER MOLLUSCA. 163 
GENUS ARION Férussac.' 
The genus Arion is dedicated 
to Mr. W. Denison Roebuck, F.I.S., 
of Leeds, whose knowledge of the 
external morphology of the British 
slugs is probably unsurpassed, and 
whose tireless exertions have so 
immensely extended our knowledge 
of the variation and distribution 
of our native species. 
The Arions (apwv, the name of 
a mythological musician and poet, 
or according to some authorities a 
mythological horse famous for its 
speed) were first imperfectly separ- 
ated from Limavr by Brard, in 1815, 
who retained the term Limaa for 
the group now called Avion, and 
constituted the genus Limacella 
for the shell only, as distinguished 
from the animal, of the species we 
now regard as the true Limaces, 
basing this separation upon the 
possession of a distinct shell in 
fimazx, and the presence of a few 
chalky granules only in Avion. Baron Férussac also separated the genera, 
using the terms Arvon_and Limaa to distinguish the groups, and basing 
the separation upon the presence or absence of the caudal gland. 
The Arions have been variously divided, Moquin-Tandon using the degree 
of firmness and coherence of the lime particles representing the shell, as 
the basis for his groups Lochea and Prolepis, while Mabille and Seibert 
distinguish sections by the terms Baudonia, Kobeltia, and Carinella. Dy. 
Simroth utilizes the modifications of the atrium, forming the groups Mona- 
tride and Miatrida, according as the vestibule remains simple or develops 
a secondary enlargement originating at the free oviduct. Signor Pollonera 
has, however, poimted out how unstable this character is, and instances 
Arion hortensis as a species in which the change from a monatriid to a 
diatrid condition can be easily traced in passing from France to Germany. 
The Monatriide, represented by Arion minimus, A. subfuscus, and A. 
circumscriptus, are said by Simroth to be characterized externally by a 
distinct band on each side of the body, while in the Diatriidw, represented 
by Arion ater and A. hortensis, the band shades away outwardly ; this 
difference is, however, not so markedly observable in British specimens, 
but in both groups there is a general tendency to become unicolorous and 
render this difference obscure and unrecognizable. 
The Arions are unquestionably a closely-allied group, in which it is 
extremely unsafe to establish new species upon examples in alcohol or 
other preservatives, and in which it 1s necessary to study the characters 
exhibited by the living animal in order to arrive at sound conclusions, as 
the internal characters of even our good and undoubted species present a 
strong family likeness, and the differences are not always very decided, 
