1 1 2 Shell Life 



the oldest portions of the shell — the shelly matter at 

 this part frequently becomes eaten away by the 

 action of gases dissolved in the water. It is about 

 3 inches long. The animal is coloured light red 

 tinged with grey, and the mantle is bordered with 

 brown. The siphons of ^phcarmm and Pisidiii'in are 

 represented in Unio by a couple of orifices on the 

 hinder margin of the mantle, of which the dark 

 brown upper (excurrent) is somewhat more elongated 

 than the lower grey (incurrent). This may be 

 regarded as an English, rather than a British, species, 

 for it occurs only in the rivers and ponds of England 

 and Wales, and does not extend north of Northumber- 

 land, or occur in Ireland. 



The Swollen Kiver Mussel (/7. tiimidus) is a slightly 

 more solid shell than the last, and viewed endwise 

 presents a much greater thickness — hence its name. 

 In colour it is a glossy brown, the lines of growth 

 often tinged with green. The interior is bluish and 

 pearly, with strongly marked impressions of both 

 mantle and muscles. The hinge -teeth, though similar 

 to those of the last named, are of a stronger character. 

 The animal differs in colour from U. j^'^ctoruon in 

 being greyish instead of red. Its distribution is 

 nearly the same as that of U. pictorum, except that 

 it does not occur north of Yorkshire. Both of these 

 species produce pearls, but they are very small and of 

 inferior quality. The Pearl Mussel is U. ^tnargaritifer, 

 which has a shell of more oblong shape, m.uch 

 flattened, though of solid material. There is no 

 gloss upon its rough, blackish-brown surface. Its 

 length is about 5^ inches. The interior is pearly, 

 but in this case the tint is flesh-colour, varied with 



