Introductory 3 1 



type, should stand first, the Snail next, and the 

 Swan Mussel last, — its structure being more primi- 

 tiv^e than that of the others, — but reversing this 

 order we propose to deal first with the more simple 

 forms. For the natural sequence of the British 

 species, proceeding from the most highly organised 

 to the lowest groups, the reader is referred to the 

 Appendix. 



