X PREFACE. 



The illustrations, with the exception of two of the engrav- 

 ings, have been drawn by myself, and I am much indebted for 

 the fidelity and skill shown by my friend Mr Charles Berjeau, 

 F.L.S., in the transference of these to wood and stone. Where 

 possible, I have employed wood-engravings in preference to 

 lithographs, as it is an advantage in studying minute structures 

 to have the representation of these along with the descriptive 

 text ; but considerations of cost have prevented my using 

 woodcuts as freely as I should have wished. Except in 

 three or four instances, where the contrary is explicitly stated, 

 all the illustrations are original, and are taken from specimens 

 and slides In my own collection, — the latter being drawn by 

 means of the camera luclda. 



Finally, I have to return my most hearty thanks, for kind 

 assistance by way of specimens or advice, or both, to Mr U. 

 P. James, Mr John M. Nickles, Mr Richard Howse, Dr Gus- 

 tav LIndstrom, Mr R. Etheridge, Jun., F.G.S., Mrs Robert 

 Gray, and Dr Daniel QEhlert. No one can be better aware 

 than I am that the present work, at best, can be regarded as 

 nothing more than a starting-point from which more extended 

 and more final investigations can be carried on ; but that it is 

 not even more Incomplete than It actually is, must be largely 

 set down to the generous help that I have received from my 

 fellow-workers. 



United College, St Andrews, 

 March 15, 1881. 



