MONOGKAPII OF BRITISTI LAST) AND FRESHWATER MOLLX'SrA. 



ir,3 



(iKNUS ARION Fonissac. 



Tlie genus Arimt is dedicated 

 to Mr. W. Deiiisoii Roebuck, F.L.S., 

 of Leeds, whose knowledge of the 

 external morphology of the British 

 sings is probably unsurpassed, and 

 whose tireless exertions have so 

 immensely extended (jur knowledge 

 of the variation and distriljution 

 of our native species. 



The Arinnx (k/koji', 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 Liriia.v by Biard, in IsiT), 

 who retained the term Lima,!- for 

 the gTOup now called Arion, a.nd 

 constituted the genus Limuvidhi, 

 for the shell only, as distinguished 

 from the animal, of the species Ave 

 now regard as the true Liriiaces, 

 basing this separation upon the 

 possession of a distinct shell in 

 Limii.r, and the presence of a few 

 chalk)' granules only in Ariun. Baron Fcrussac also separated the genera, 

 using the terms Arion^&ii<\ Linui.v to distinguish the groups, and basing 

 the separation upon the presence or absence of the caudal gland. 



'Y:\\B Arions have been variously divided, Moquin-Tandon using the degree 

 of firmness and coherence of the lime paiticles representing the shell, as 

 the basis for his groups Lachea and Frokpis, wdiile Mabille and Seibert 

 distinguish sections by the terms Baudonio, Koheltin, and Carimdla. Dr. 

 Simroth utilizes the modifications of the atrium, forming the groups Moiiu- 

 triidce and iJkdriidw, according as the vestibule remains simple or develops 

 a secondary enlargement originating at the free oviduct. Signer Polhmera 

 has, however, pointed out liow unstable this character is, and instances 

 Avion hortensis as a species in which the change from a monatriid to a 

 diatriid condition can be easily traced in passing from France to Germany. 



The Monatriidic, represented by Arion minimus, A. subfuscus, and A. 

 circumscrijifus, are said by Simroth to be characterized externally by a 

 distinct band on each side of the body, wdiile in the Diatriidu', 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 Avians are unc[uestionably a closely-allied group, in which it is 

 extremely unsafe to establish new sijecies upon examples in alcohol or 

 other preservatives, and in which it is 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 A-ery decideil, 



