INTRODUCTION. 33 



The mere inspection of this table will show that 

 most of the species have a very extensive range, having 

 been found in all parts of our island where there has 

 been a zoologist who has taken any interest in 

 searching for them. Thus we find, that out of the 

 128 species recorded in this work, 107 species are 

 found within a circle of about 15 miles of the metro- 

 polis, and 89 in the northern district of England. 



Of the species which are confined to the southern 

 half of the kingdom there may be noticed among the 

 aquatic kinds — 



Assiminia Grayana, only found in the Thames 

 and the streams running into it, from its mouth to 

 where the water is only slightly brackish, or nearly 

 fresh, at the very highest tides, as at Greenwich, for 

 example. 



Paludina vivipara and P. achatina are not found 

 in the northern part of the island. P. crystallina is 

 found in the rivers of Cambridgeshire, Oxfordshire, 

 Essex, and Suffolk; and P. achatina is very common 

 in the Thames, and also in the rivers of some of the 

 before-mentioned counties : they are found together 

 in the river Colne, at Uxbridge. These species 

 avoid the slightest degree of brackishness in the water, 

 and are therefore only found in the upper part of 

 the rivers. 



Bithinia ventricosa is very commonly found with 

 this latter in the Thames, and in Suffolk; but it 

 keeps in the smaller streamlets, and is not so parti- 

 cular about the absence of brackishness, for it is found 

 with B. impura and Assiminia in the streams of the 

 Greenwich marshes. 



Neritina Jluviatilis, and Planorbis corneus, are also 

 c 5 



