20 THE SCOTTISH NATURALIST 



matters, probably not only the amount of calcareous com- 

 pounds that it contains, but also the amount of humous acids 

 present and its power of retaining moisture. 



Minor differences in the environment afforded to molluscs 

 by the various islands may also be noted. lona, Eorsa, and 

 Innis Kenneth are treeless, or practically so, while a con- 

 siderable area on Ulva is occupied by hazel-coppices, beech- 

 woods, and mixed plantations of oak, sycamore, pine, etc. 

 On Eorsa there are high cliffs with a northerly exposure, 

 dripping with moisture, and draped in a peculiarly luxuriant 

 vegetation of ferns, mosses, and other plants, so rich in certain 

 small caves that it has almost an artificial appearance. On 

 the other islands the cliffs are drier and barer owing to 

 differences in exposure. Ulva, however, owing to its greater 

 area and higher hills, is much richer in small streams than 

 the other islands, most of which, indeed, can hardly be said 

 to possess real streams. The few pools of stagnant fresh- 

 water on Ulva are situated at comparatively high altitudes, 

 mostly on the hill-tops, and are brown and peaty and often 

 choked with Sphagnum, whereas those on Innis Kenneth, 

 which are formed by little springs on the sandy slopes above 

 the beach, are full of clear water. Both lona and Innis 

 Kenneth contain ancient ecclesiastical ruins, very attractive 

 to certain snails, and differing from the comparatively recent 

 ruined cottages on Ulva in that their stones are fixed 

 together with mortar on which a kind of calcareous deposit, 

 resembling that which forms stalactites and stalagmites in 

 caves, has developed in the course of ages. 



What, then, are the differences in molluscan fauna 

 associated with these differences in physical environment on 

 these small islands, situated so near one another on the 

 eastern side of the Atlantic? The most easily explained is 

 the absence or great scarcity of true arboreal forms on all 

 but Ulva, in the hazel-coppices and beech-woods of which 

 Balea perversa is abundant on tree-trunks. A more striking 

 and perhaps more interesting difference, but one much 

 less easily explained, is the abundance on lona and Innis 

 Kenneth of the conspicuous Helicidae Helix aspersa and 

 Helicella itala (or ericetorunC) and their absence from the wet 



