Mr. Alder's Catalogue of Land and Fresh-xmter Shells. QTJ 



presents an undulated surface of hill and dale, frequently intersected 

 by deep wooded dells, locally called deans. In these last, affording a 

 plentiful supply of shade and moisture, the greater part of the land 

 Mollusca are usually found : some species inhabit the roadsides and 

 hedges ; while a few, and only a few, are found in the more elevated 

 situations. Many species occur in great abundance on the limestone, 

 some of which are not to be met with in any other part of this neigh- 

 bourhood. These are generally such as prefer a dry situation. Whe- 

 ther it is on this account only that they are found so abundant on this 

 rock, or whether there is something in the limestone and its peculiar 

 plants which is favourable to their growth and re-production it is diffi- 

 cult to determine. The quantity of lime necessary for the formation 

 of their shell is secreted by these little animals in all situations, but it 

 is not improbable that the secretion may be more readily effected where 

 this material abounds. 



Our sea banks also afford a favourite residence for numerous species. 

 Of these the Pupa marginata, which is common amongst the scanty 

 herbage of our sandy links, seems peculiar to such situations in this 

 neighbourhood. Our rivers and streams are, for the most part, too 

 rapid to be suitable for the habitation of the fluviatile Mollusca, and the 

 quantity of marshy land in this part of the country is but small. As 

 may be expected, therefore, the fresh-water species are not very nu- 

 merous. Many which are of common occurrence in other places, are 

 here very rare ; Paludina impiira, for instance, a shell which abounds 

 in almost every ditch in many parts of England, is here a rare species, 

 confined to one or two localities ; while Planorhis complanatus and con- 

 tortus, Lymnxa stagnatis, and Physafontmalis, also common in the sou- 

 thern counties, have been here found only in one spot — Prestwick Car. 



The proportion of the land to the fresh-water Mollusca in the British 

 Fauna appears to be about 5 to 4. In Dr. Fleming's British Animals, 

 the latest work on the subject, there are described, exclusive of the 

 LimacidcB, 94 species ; of these 53 are land, and 41 fresh-water. Mr. 

 Miller, in the Annals of Philosophy, enumerates 60 species found in 

 the environs of Bristol, 32 of which are land, and 28 fresh-water : and 



