368 CONCLUDING REMARKS. 



extends from the edge of the Grampians northward, do 

 not exceed thirty ; and yet those which I have found in 

 our Aberdeenshire portion of it alone, amount to forty- 

 two, or a fourth more. According to him also, no 

 species of Planorbis or of Physa, occur in the North of 

 Scotland ; yet five species (taking Spirorbis as one) of 

 the former, and one of the two British species of the 

 latter, are found abundantly in Aberdeenshire, within a 

 circle of two miles diameter. But, although there is 

 some inaccuracy in the statements relative to Scotland, 

 the essay alluded to is remarkably interesting, and indi- 

 cates an extensive knowledge of the subject. 



The Terrestrial Mollusca of our district increase in 

 the number of species, and in that of individuals, from 

 the mountainous parts of the interior, toward the lower 

 tracts bordering on the sea, and are especially abundant 

 by the larger rivers, and on the grassy slopes of the coast. 

 Helix aspersa nowhere occurs far inland, and Helix 

 arbustorum and hortensis have not been found by me 

 extending beyond thirty miles from the sea-coast. 

 Although the nature of the rock may influence the num- 

 ber of individuals, it does not, with us, affect the number 

 of species. It is remarkable that our sandy links or 

 downs are entirely destitute of Helix ericetorum and 

 Bulimus acutus, which in similar tracts, are abundant 

 on some of the western coasts of Scotland. 



The Fresh-water Mollusca also increase in frequency 

 from the interior toward the coast ; but in low tracts, 

 they are as abundant far inland as near the sea, which 

 thus appears to have no influence upon them. Limneoe, 

 Planorbes, Ancylus fluviatilis, and several species of 

 Pisidium, are very abundant in the lower tracts, and some 

 of the latter genus are plentiful among the sphagna and 

 other plants of wet moors. The nature of the subja- 

 cent rock appears not to have any direct influence upon 

 them. 



Our estuaries are less productive than might be ex- 

 pected. Those of the Dee and Don are remarkably 

 steril ; but that of the Ythan, which is larger, and more 



