UNIO. 35 



similarly arranged, but they are finer, sharper, and more erect: 

 muscular scars distinct: pallial scar faint, owing tothegreater 

 thickness of the nacreous lining. L. 1'33. B. 3. 



Var. 1. radiata. Shell having faint and narrow rays of 

 green which diverge from the beak. 



Var. 2. curvirostris. Shell smaller, shorter, and flatter: 

 epidermis yellowish-green, with brown zones : posterior side 

 curved and wedge-shaped. U. curvirostris, Normand. 



Var. 3. latior. Shell broader and shorter, yellowish-brown. 



Var. 4. compressa. Shell very broad and flat; upper margin 

 raised and curved : posterior side greatly compressed and at- 

 tenuated, assuming a beak-like form, and having a double 

 ridge and furrow which runs from the beak in the younger 

 state of growth : lower margin straight : lunule broad, and ex- 

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

 other. 



HABITAT : Rivers, ponds, and canals throughout En- 

 gland ; but it does not appear to have been found north 

 of Yorkshire. It is also one of our upper tertiary fossils. 

 It ranges from Finland to Algeria and Sicily. Var. 1. 

 R. Avon, Bath (Clark). Var. 2. From Clark's and 

 Mrs. Loscombe's collections of British shells, but with- 

 out any indication of locality. Var. 3. Canal near 

 Oxford (Whiteaves). Var. 4. Norwich (Bridgman). 

 This remarkable form might easily be raised to the 

 rank of a distinct species ; but I can only regard it as 

 abnormal, and analogous to the variety ovalis of U. tu- 

 midus. 



This species was confounded by Lister, Linne, Miiller, 

 Draparnaud, and all the older writers with U. tumidus. 

 It differs from that species in the form of the shell, which 

 is oblong instead of oval ; in its much greater propor- 

 tionate breadth ; its thinner texture ; in the upper and 

 lower margins being nearly straight and parallel, instead 

 of being curved and wedge-shaped; in the umbonal 

 region being much less prominent and swollen ; and in 



