162 



MONOGUAPII OF DRITISII LANP ANIi FRESIIWATEI! MOLLUSCA. 



Famti.y AKTOXTD-E Gray. 



The family Arioiii(hr, according to Pilsbiy, 

 is souiewliat (liscoiitimious in its geogTa|)liical 

 range, occnp3'ijig three widely separated areas, 

 in each of which a predominant type occurs. 



The West American area, has the greatest 

 nunihcr and variety of genera, embracing the 

 Hiruiiii'niii , A /■/iiI/niKi-iiiif, etc., and forming 

 the nidst ]iiiniitive grou]i of the family; one 

 firni, />'/':/ III 1/(1. ]iossessing a spiral external 

 shell with sculptured nepionic wdiorl, short 

 liody cavity, and solid tail, may be regarded 

 as linking Ariiii)lil<( with the Endodonfldw, 

 from which they are supposed to have been 

 derived. 



The ^Vsiatic centre is concentrated in the 

 Ilinialayas, and represented by Anadenua, a 

 group in which the caudal gland is wanting 

 and the male intromittent organ still present, 

 undoubtedly representing in these respects the 

 must ancient form of the group. It is most 

 closel}' allied to the genus Frojdnjsaon, but in 

 the jienial development shows nearest athnity 

 to Hc^jii'mriiiii, both ol which are now West American groups. 



The true Arian^, the most highly developed forms, have their home in 

 the European region, the probable s(.iurce of origin of the entire group, from 

 whence in past ages the earlier and more primitive genera have spread 

 throughout the northern hemisphere, the most simply-organized groups, as 

 is usual, occupying the regions most remote from their place of origin, and 

 not, as is to(.i ])revalently believed, persisting in their evolutionary centre. 

 The Ari<i)is are remarkable for the peculiar jienial degeneration they have 

 undergone, and the assumption of the intromitteut function of that organ 

 by the o\'iilucal passage. 



The A riiii/iihr are not descended directly from the ]U'iniitively shell-less 

 forms, as has lieeu averrcil, but iinmistakeably show their descent from a 

 group with well-de\"('lo|icd spiral shells, the American forms supplying the 

 chief links which make plain the progress of the modifications and clearly 

 ilemijiisir.-itc that the typical genus Ar/oii is the terminal member of a 

 scries of toruis lic^inuiiig with B/iiiii'ifa, half-slug and half-snail with 

 alnujst hclicoid nnisculaturc, and passing by numerous intermediate stages 

 stdl I'xi^hiiu; lo till' ty|iii','il ^\r/o» organization. 



'J'ho laiMily IS )iroliably most satisfactorily dixidctl by utilizing the various 

 raclor muscles, (heir .arrangement showing also 



<^t/i 7i//o**€>t 



Alt 

 I lie wi'i 



Wilt 

 (■oi'(h,'d r( 



I loiLN ol t ho TriT 



III ics( iliricrciicrs lii'l wren 



ibis uroiip Sigiior ( 'arlo 1 



gnilioji of tllo rxtollt ,'11 



I, s]iecilir dillrinil |;il ii 



ciici;ill\'. 



Ariimiilir and other slugs, 

 luer.-i, of Turin, is here associated in 

 iM]iorl.-iiicc of his researches upon the 

 and rl;issilicalioii of the Arloiiic/ic 



ov^;illl/,al loll, 

 ;iihI of I lie si 



III llir Hiilish Isles III,, liiiiiily is iv|iresriiled by only two genera, .-I /■/, 



:itid (unii/iifiirii.^, Ihe ri'iii.-iiiiiiin' and i v ancient ' i;rou|'is bein 



restneteil to the remoter jiaits of Ihe iioitliern heniispliere. 



on 

 now 



