12 UNION ID^. 



respects resemble their marine allies the lobster, crab, 

 shrimp, &c. 



They are active in their habits during the summer 

 months, but in winter bury themselves in the mud. 



GENUS I.— UNIO,'' PHILIPPSSON. 



Body long, somewhat swollen ; gills (lower orifice) almost 

 straight. 



Shell long, solid ; lateral teeth strong ; lunule distinct. 



In Great Britain three species only, belong to this 

 genus ; they are all oviparous, and produce pearls. 



The portion of the shell which surrounds the beaks 

 (or ttmbones) is frequently eroded, or worn away. 

 Gwyn Jeffreys believes that this is ''caused by the 

 chemical action of gases which are evolved from the 

 mud in which this portion of the shell is usually im- 

 bedded." — B.C., vol. i. p. 32. 



I. UNIO TU'MIDUS,t PHILIPPSSON. Pl. I. 



Body greyish ; foot milky-grey ; mantle edged with brown ; 

 upper orifice elongated, brownish ; lower orifice pale grey, some- 

 times tinged with orange-brown. 



Shell oval, convex above, swollen, solid, glossy, of a brown 

 colour, often tinged with green in the line of growth ; epidermis 

 strong; beaks somewhat incurved, not central, surrounded by 

 wavy folds which are often pitted ; lumile narrow, lance-shaped ; 

 liga7nent strong, short, prominent ; anterior side rounded, sloping 

 downwards ; posterior side sloping to a point ; lower margin 

 curved ; inside bluish-white, pearly ; hinge strong ; on the an- 

 terior side of the right valve there is a strong tooth forming 

 with the margin of the shell a groove into which a corresponding 

 notched tooth on the other valve fits ; on the posterior side 



A pearl. f Swollen. 



