The Scottish Naturalist. 247 



Hawaiian Islands affords many notable illustrations of this. In 

 these cases there is no doubt that other causes than the geological 

 formation influence the distribution ; but it is evident that the 

 formation has no little effect upon the range of land Mollusca ; 

 and it is a cause that should not be overlooked, as it possibly is 

 sometimes, in theorising on the subject. 



In the richly-wooded and damp ravine that forms the lower 

 part of Glen Tilt, Mollusca seem to be abundant. Perhaps 

 the most interesting, as being rather scarce elsewhere, though 

 abundant in this spot, and, moreover, a species whose centre 

 of distribution seems to be, contrary to the usual rule. Western 

 instead of Central or Eastern Europe, is Helix fusca. 



Opposite Forest Lodge, on the calcareous slopes of Craig Mohr, 

 Mollusca, as remarked above, abound. This was never more 

 noticeable than on one wet afternoon, when, having nothing else 

 to do, I climbed the very steep, nearly precipitous bank, that, to 

 a height of about 600 feet, overhangs the Tilt at this place. I 

 believe I do not overrate the number of snails when I say, that 

 on an average every square foot of this slope had at least one 

 snail upon it, and often two or three. All these snails belonged 

 to two species, of which Helix arbustorum was very much more 

 abundant than the other, H. nemo?^alis. I went over about 

 half-a-mile of the bank, and as the snails were equally abundant 

 everywhere, I am probably very much under the mark when I 

 calculate the total number as about two millions. The slope 

 continues to be of the same character for about a mile and a 

 quarter, and there seems every probability that the snails were 

 equally abundant over the whole of it, which would give a 

 total of five or six millions of those two species. Most of the 

 Helix arbustorum belonged to the variety alpestris, but some, 

 though in size the same, have depressed (or rather, have not ele- 

 vated) spires. Without exception, I think all the Helix 7ie?noralis 

 had yellow, unhanded, white-lipped shells, rather smaller than 

 the usual var. horte?isis, and approaching in size var. minor. In 

 the ravine of one burn I found a few H rotundata, but ex 

 cept in being a little smaller, the specimens do not differ from 

 those found elsewhere. They occurred at an altitude of about 

 1750 feet, in company with Zonites crystallintis, Z. nitidus, Z. 

 alliarius, Vitri7ia pellucida, Helix concinna, and Limnoea trun- 

 catula. 



On some of the rocks, ascending to about 1600 feet, Buli- 

 mus obscurus was not uncommon. The shells are rather larger 



