246 The Scottish Naturalist. 



it. Gelechia soluteila, populella, galba?iella, &c., occur in various 

 parts of the glen, and CEcophora simidella in the fir woods. Saxi- 

 fraga aizoides being such a common plant, Zelleria saxifragce of 

 course abounds, living not only upon the above-mentioned saxi- 

 frage, but also on S. oppositifolia. From larvae found on the 

 under-side of the leaves of He7\iclewn, I reared some unusually 

 dark specimens of Chaidiodus chcei'ophyllelhis. 



Of the ''plumes," Aciptihis tetradactylus is excessively abun- 

 dant, and Pterophoriis fiiscus dca^ plagiodactylus occur here and 

 there. 



Two specimens of the rare brachydactylus have been taken near 

 the mouth of the glen. 



Of the other orders of the Insecta I will not on the present 

 occasion say anything, but proceed to give a short account of the 

 MoUusca of Glen Tilt. 



MOLLUSCA. 



The land-shells are chiefly interesting from the manner in 

 which they are distributed in the glen. In many parts of the 

 glen snail-shells are so rarely met with, that they become con- 

 spicuous by their absence. In other parts they are conspicuous 

 for the very opposite reason, that they are so abundant as to com- 

 pel observation. In no place can this be seen better than in the 

 immediate neighbourhood of Forest Lodge ; for there the river 

 forms a sharp line of demarcation between ground swarming with 

 shell-clad Molluscs and ground where scarcely a specimen can 

 be seen. It would seem as if a devouring army of snails had 

 marched over the hills from the south, till they had been stopped 

 by the river. Though the stream would doubtless serve as an 

 effectual barrier to the snails, if they depended on their own exer- 

 tions to cross it, yet their stay on the south side of the valley at 

 this spot is due to a very different cause. The south side of the 

 glen here is composed of limestone, the north of gneissose or 

 quartzose rocks. The plants of the south side provide the snails 

 with abundance of that calcareous material, out of which their 

 shells are made ; the plants of the north contain little or no lime. 

 And the same cause influences the distribution of the Mollusca 

 in other parts of the glen, but in few places is it so markedly 

 shown as at Forest Lodge. 



Land Molluscs are notorious for their eccentricities of distri- 

 bution — many species being limited to a single island, valley, or 

 even to a single rock. The rich conchological fauna of the 



