1 6 UNIONID&. 



erosion about the beaks, and the posterior teeth are 

 much less fully developed. The interior of the shell 

 is very thickly coated with mother-of-pearl. 



Pearls occur much more frequently in this species 

 than in U. tumidtis or U. pictorum, and are usually of 

 a larger size and of greater value than those produced 

 by them ; their prevailing colour is white, but green, 

 brown, black, and flesh-coloured or pink ones are 

 occasionally found, those of the latter hue, when large 

 and well shapen, being of considerable value. At 

 one time the pearl fisheries in this country were a 

 source of considerable revenue to their owners ; it is 

 stated in Brown's ' Recent Conchology ' that the pearls 

 sent from the River Tay, in Perthshire, to London, 

 from the year 1761 to 1764, were worth io,ooo/. 



Var. i. sinuata. Shell broader than the typical form, yel- 

 lowish-brown ; lower margin concave in the middle. It occurs 

 in some of the streams in the West of Scotland. I have met 

 with it in the River Clouden, near Dumfries ; it is also found in 

 the West of Ireland. Perthshire (F. B. White), S.N. 



Var. 2. Roissyi, Michaud. Shell proportionably longer, lower 

 margin convex or rounded. Yorkshire (Sowerby), B.C. Perth- 

 shire (F. B. White), S.N. 



GENUS ILANODONTA* LAMARCK. 



Body somewhat oval ; mantle with thickish and fringed edges ; 

 gills flexuous. 



Shell oblong, thin, somewhat compressed anteriorly, beaks not 

 very prominent ; hinge almost toothless ; scars indistinct. 



The Anodontce are ovoviviparous, in which respect 

 they differ from the Uniones, but their habits are very 

 similar. 



* Toothless. 



