38 UNIONID.E. 



strong ; the right valve having on its anterior side a very broad, 

 thick, and blunt double tooth, the crest of which is irregularly 

 tubercled, and on its posterior side a blunt and obscure ridge- 

 like plate, which in young specimens is grooved or double ; 

 left valve furnished at its anterior side with a single, conical, 

 strong and blunt tooth which locks into the double tooth, the 

 posterior tooth in this valve being similar to the corresponding 

 one in the right valve : muscular and iiallial scars very deep 

 and distinct. L. 2-4. B. 5. 



Var. 1. sinuata. Shell rather broader in proportion to its 

 length than in the type, yellowish-brown : lower margin in- 

 curved towards the middle. U. sinuata, Lam. Hist. An. s. 

 V. vi. pt. i. p. 70. 



Var. 2. Roissyi. Shell proportionably longer : lower marc/in 

 convex, or rounded. U. Roissyi, Michaud, Compl. p. 112. 

 pi. xvi. f. 27, 28. 



Habitat : Mountain rivers and streams throughout 

 the British Isles. It is found in several parts of the 

 Swansea Canal where the bottom is gravelly, having 

 been carried in by the water-courses which supply it. 

 It also ranges through the mountainous and hilly parts 

 of the Continent from Lapland to the Pyrenees. Var. 1. 

 West of Scotland (Bedford and J. G. J.) ; West of 

 Ireland (Humphreys and Barlee). A specimen of this 

 form from Co. Kerry measures nearly 6 inches in 

 breadth or width. Var. 2. Yorkshire (Sowerby). A 

 monstrosity also occurs having a longitudinal ridge in 

 the middle of the shell. 



This species differs from all the others in its shell 

 being much longer (measured from the beak to the lower 

 or front margin) as well as more depressed, in its dull 

 aspect and much darker colour, the extensive erosion of 

 its umbonal region, and especially in the posterior teeth 

 being scarcely developed. 



The lining of mother-of-pearl is equal to half the 

 entire thickness of the shell, as may be seen by grinding 



