XXll INTRODUCTION. 



Trochus alahastrum, helicinus and undulatus, and Velutina 

 flexilis. A considerable portion of our Ascidians belong to 

 this type. 



IX. A. few of our most northern shells may be regarded 

 as belonging to a still more northern, an Arctic type. 

 Astarte arctica, Natica pusilla, Scalaria Granlandica and 

 Terehratula cranium, may be accepted as representatives 

 of this group. 



Our land and freshwater Mollusks may likewise be 

 grouped under several types, which, however, we shall 

 sufficiently indicate hereafter when noticing their foreign 

 relations. 



The main text of this work is occupied with the descrip- 

 tions and history of the species which are true and indi- 

 genous members of these several types. Only such can 

 fairly be considered as British Mollusca. In this division 

 of the animal kingdom, unlike what may happen in the 

 higher and more mobile sections of our fauna, there is 

 little danger of the intrusion of stragglers, although a few 

 of oceanic habits, or else transported through the uncon- 

 scious agency of man, acting as their carrier through the 

 medium of his sea-traversing ships, do find their way 

 legitimately into our seas, A few land-snails also trans- 

 ported in timber, or about the roots of plants, have esta- 

 blished themselves, though only for a limited period, on our 

 soil. Such legitimised foreigners we have, in almost every 

 instance, described in a supplementary shape at the end of 

 the genera to which they belong. A residue of unlawful 

 and confusing intruders remains to be accounted for. 



The number of exotic shells erroneously introduced into 

 our fauna is very large, our earlier writers having frequently 

 considered them as indigenous upon very slender grounds of 

 evidence. When once the species was inserted in our lists, 



