INTEODUCTIOK. XIX 



the large majority of the remainder must have been so small as to 

 be commercially worthless ; there is no doubt, however, that the 

 whole industry is of considerable value. 



There is also a legend, which may or may not be true, that one 

 of the reasons which tempted Caesar to the invasion of Britain 

 was the exaggerated stories current in Borne of the magnificent 

 pearls produced by the river mussels of these Islands ; whether 

 this is true or not, the fact remains that, certainly up to a few 

 years ago, the Highland peasants were in the habit of eking out 

 their scanty incomes by fishing for Margaritana margaritifera in 

 the Scottish rivers, though the pearls obtained therefrom were 

 not of great value, they being generally small and of a dark 

 colour ; but the shell itself does not seem to have been a com- 

 mercial article, possibly because of the small quantities only 

 obtainable. 



In China an extensive artificial cultivation of " blister pearls " 

 was at one time in vogue, the mode of culture being as follows. 

 When adult, or nearly so, a large species, Dipsas plicata, was 

 selected ; a wedge was placed temporarily between the valves in 

 order to prevent their closing, while obstructions, often in the 

 shape of small metal images of Buddha, were inserted between the 

 mantle and the shell, generally in horizontal rows, the irritation 

 caused by these foreign bodies in the shell induced the animal to 

 cover them over with a layer of shelly nacre, though how long 

 the whole operation took before they were fully and sufficiently 

 thickly coated it- is difficult to say; as many as twelve of these 

 metal images were frequently placed in a single shell, each image 

 measuring about 19 by 13 millimeters. 



In India and the middle East, however, the author has never 

 heard of any economic use for the freshwater mollusca, and 

 certainly, after a sojourn of some years in Ceylon and Southern 

 India, no case of this ever came to his notice. 



In conclusion the author's thanks are due to many friends for 

 help most willingly given in various ways, among whom may he 

 mentioned Messrs. E. A. Smith, I.S.O., B. B. "Woodward, E.L.S., 

 and Gr. C. Hobson, of the British Museum, Lieut. -Colonel H. H. 

 Godwin-Austen, E.R.S., Mr. T. Iredale, and especially to Dr. N. 

 Annandale of the Indian Museum, Calcutta, who has been kind 

 enough to place the whole of the Indian Museum Collection of 

 Naiades at his disposal in order to assist him in his work. 



To Mr. Eorster Cooper, of the Museums at Cambridge, the 

 author is also indebted for the loan of certain specimens from the 

 Benson Collection for purposes of figuring. 



Eurther, the author would wish to acknowledge the help 

 afforded to him in compiling the above binomic notes by 

 Mr. B. B. Woodward's admirable book 4 The Life of the Mollusca/ 



53 West Cromwell Road, S.W. 

 March 1915. 



