22 THE SCOTTISH NATURALIST 



were taken on the Ross of Mull. In the little deltas of the 

 Ulva streams the \iXz.cV\^\\-^dX&x Hydrobiaiilviz-^^.'s, abundant. 

 L.peregra and P . personatiim exhibited interesting differences 

 on the different islands and in different parts of Mull in cor- 

 relation with environment. 



Considered as a whole the inland molluscan fauna of 

 Ulva and the other islands is far from being highly 

 specialised. It consists rather of species and genera of very 

 wide geographical range — species, that is to say, that are 

 physiologically adaptable to different conditions of life. 

 Conditions are here unfavourable to large and conspicuous 

 forms that bear shells. They are, on the other hand, favour- 

 able to certain slugs, particularly to the Common Black Slug, 

 y^rz(3« «^^r, which, being indifferent to the presence of humous 

 acids, is extremely abundant in the bogs of Ulva. It is an 

 interesting fact that, dependent as the land-molluscs are 

 on moisture, species of land-snails are far more rich in 

 individuals in dry than in damp situations. In this country 

 it is mainly on sand-dunes that individuals of the Helicidse 

 abound, and in the Mediterranean region, and especially in 

 the coastal districts of North Africa, every dry tuft of grass 

 and the twigs of every desert shrub are often so thickly 

 covered with snails that they have the appearance at a 

 distance of being in full bloom. Such species belong mostly 

 to specialised genera, such as Xerop/n/a, with thick white 

 shells. Other genera, or species (usually with thin trans- 

 lucen.t shells), which are never so prolific, seek damp 

 situations and shun light; for example, all species of 

 Hyalinia, and among the Helicidae, Pyraniidula rotundata. 

 It is such species that, among the shelled forms, constitute 

 the bulk of the inland molluscan fauna of the Hebrides. In 

 the islands under consideration the commonest snails are 

 H. cellaria and P. rotundata. 



The following table gives the names of the species 

 found, and indicates the islands on which they were 

 collected. In nomenclature I have followed Taylor's Land 

 and Freshwater Molhisca of the British Isles so far as it is 

 published. When this has failed me, I have used the names 

 in Roebuck's " Census" {Journ. Cofwh., June, 1921). 



