276 JOURNAL OF CONCHOLOGY. 



VARIATION IN LEHMANNIA ARBORUM, 



THE TREE-SLUG. 



By WM. DENISON ROEBUCK, F.L.S. 



[Read before the Conchological Society, December 4th, 18S4.] 



In continuation of my intention of bringing together for the use 

 of EngUsh collectors the various descriptions of the varieties of 

 slugs which have been characterized by authors, I now lay before 

 you those of the tree-slug. 



For most of the varieties it will be seen that we are indebted 

 to the Italian malacologists, and particularly to Lessona and 

 Pollonera, as given in their magnificent ' Monografia dei 

 Limacidi Italiani.' 



In that work they offer some remarks bearing on the 

 influence of climate upon variation which it will be of interest to 

 reproduce. They state that the length of the keel is very 

 variable, and that it is generally short in the individuals which 

 inhabit the lower regions, but longer in those of the elevated 

 regions, and sometimes so much as to reach to almost four-fifths 

 of the length of the back. Generally this lengthening of the keel 

 is accomi^anied by obscuration of the colouring, so that the 

 individuals found in the most elevated regions of the Alps of 

 Piedmont seem practically black [var. rupicohi\ the usual light 

 length-bands being invaded by a blackish tint to so great an 

 extent as to render them practically obsolete. 



My own experience of the present species tends to bear out 

 these views. It appears to be in Britain a decidedly northern 

 species, for although it occurs in every part of the British Islands, 

 it is to be regarded as being more a sporadic and woodland 

 species in the south, while in the north of England it becomes 

 of wider range, and in the north of Scotland it is one of the 

 commonest forms of slugs, surpassing Z. agrestis (which is more 

 addicted to cultivated land) and almost rivalling Arion ater in 

 abundance. I have also noticed that the Scottish specimens are 

 much darker and not so translucent as the southern English. 



J.C, iv., Jan., 1885. 



