UN 10, 15 



state of growth ; lower 7nargm straight ; lunula broad, and 

 extending between the beaks, so as to separate them from each 

 other. Norwich (Bridgman), B.C. 



3. U. margari'tifer,* Linn]^. Pl. IL 



Body dirty grey with more or less of a reddish tinge, some- 

 times of a Hvid flesh-colour ; foot large, tongue-shaped, somewhat 

 obtuse, yellowish-grey or dirty red ; mantle bordered with grey 

 in front ; orifices brownish-grey with whitish rays. 



Shell oblong, compressed, solid, not glossy, blackish-brown, 

 strongly and roughly striate in the line of growth ; epidermis 

 thick ; beaks placed away from the centre, incurved ; twibonal 

 region eroded so deeply that the under layers of the shell are 

 exposed ; hinule indistinct ; ligament very long, produced to 

 the anterior side ; hi?tge line curved ; anterior side rounded ; 

 postei'ior side sloping gently, rounded at the end and bluntly 

 keeled above ; lower margin straight or slightly concave ; itiside 

 tinted with flesh-colour, pearly, stained with dirty green about 

 the adductor muscles, and pitted in the middle ; hinge strong, 

 the anterior side of the left valve is furnished with a broad, strong 

 double tooth tubercled at its tip, the posterior side has an in- 

 distinct ridge ; on the anterior side of the right valve there is a 

 strong cone-shaped single tooth which fits into the double tooth 

 just described ; on the posterior side there is a ridge-like plate 

 similar to that in the left valve ; muscular ^ndpallial scars very 

 strong and deep. 



Inhabits rivers, chiefly in the mountainous districts 

 of Great Britain. Gwyn Jeffreys says it occurs " in 

 several parts of the Swansea Canal, where the bottom 

 is gravelly, having been carried in by the water- 

 courses which supply it." This species is longer from 

 the beaks to the lower margin than the two preceding 

 ones, and is more compressed, of a darker colour, and 

 less glossy surface ; it is also subject to much greater 



Pearl-Learim 



