ON BRiTISH PULMONIFEROUS MOLLUSCA. 129 



Though geographical and geological causes mainly regulate 

 the distribution of Pulmoniferous Mollusca, there are circum- 

 stances which modify their influence in certain situations. One 

 of the most important of these is elevation. In some countries, 

 as in Switzerland, the influence of elevation is positive ; that is 

 to say, certain species occur at certain heights which are not 

 found at a lower level. In Britain the influence of elevation is 

 merely negative, the ascent of our mountains being charac- 

 terized by the absence of species, the species becoming fewer 

 as we ascend : towards the summit we find only Helix al- 

 liaria. The neighbourhood of mountains also affects a fauna, 

 the upland species predominating around their bases even 

 when the hills rise directly from the region of the plains. A 

 wooded district is peculiarly favourable to the multiplication of 

 land shells, especially the smaller Helices, Pupae, and Clau- 

 siliae, many of which we need not look for except in forests. 

 The species of trees which grow in woods must also be re- 

 garded, pinewoods especially being much more unfavourable 

 to a fauna than woods of any other kind. The presence of 

 the aquatic Pulmonifera in a province must of course depend 

 on the presence of water in the various forms of lake, river, 

 ditch, and canal, each being characterized by its own peculiar 

 species. The introduction of canals into a district must mate- 

 rially change the character of its aquatic fauna ; and there are 

 shells in the British lists, such as the Dreissena jpolymorpha, 

 which owe their presence almost entirely to the construction 

 of canals in their several localities. The nature of the beds 

 of the lakes, streams and ditches in a district, affects the 

 number and variety of species therein found. We need not 

 look for many Limnei or Planorbes on gravelly bottoms, or 

 for Ancylus on mud. The presence of many aquatic mol- 

 lusca is determined by the plants growing in the localities they 

 frequent. The neighbourhood of the sea exercises a most im- 

 portant influence on our fauna, both as regards the species and 

 the multiplication of individuals. The marine influence would 

 appear to be especially favourable to the propagation of spe- 

 cies. The shells found on sea-banks are generally found in 

 great numbers, as we see in the case of Bulimus acutus. Helix 

 ericetorum, Helix virgata, Pupa marginata, and others. This 

 is especially seen in the case of species common inland as well 

 as near the sea, as Vitrina pellucida, and Bulimus lubricus. 

 The presence of sand aids this multiplication of individuals, 

 but is not indispensable. The marine influence appears to be 

 favourable to the development of size and colour in a species. 

 Helix ericetorum is generally found larger on the sea-side than 



1839. K 



