MTTSSBLS. 193 



Pisidiwrn roseum is also fottad in weedy ponds and ditches. 

 The shell is oblong, and is about ^in. in length, but in other 

 respects it is very like that of the last species. It receives 

 its specific name from the rose-colour of the upper part of 

 its body. 



The TJnionidoB are more commonly called Mussels than any 

 of the other fresh-water bivalves. Some of them are ovo- 

 viparous, while others are oviparous. They live in rivers, lakes, 

 and ponds throughout Britain, and they feed upon the 

 vegetable and animal matter which they extract from the 

 water, as well as upon Entomostraca and other minute animals. 

 Each individual of this family is both male and female. In 

 winter they bury themselves in the mud. These animals, 

 instead of having one or two siphons, like the SphceriidcB, 

 have two orifices. The lower orifice, which is bearded, is used 

 for respiration, and the upper orifice, which is beardless or 

 simple, serves for getting rid of that from which the animal 

 has extracted all nutriment. The foot is . large and tongue- 

 shaped. 



The Unionidce are divided into two genera, viz.; (1) Unio; 

 (2) Anodonta. The former genus possesses two species, and 

 the latter only one. 



The Uniones are oviparous, and sonae of them contain pearls ; 

 hence the generic name of Unio. 



Unio tumidus (Pig. 128) is a handsome bivalve, found in the 

 sluggish and stagnant water of nearly every part of Britain. 

 It is occasionally sold by dealers in aquarium necessaries, as the 

 " Duck Mussel." It is useful and interesting in the tank, where 

 at times it is rather active in its movements, leaving long 

 grooves behind it in the sand and gravel, as it travels from 

 spot to spot. The upper orifice is a short brownish tube, and 

 the lower or branchial orifice is somewhat lighter in colour. 

 The shell has a swollen appearance (hence the specific name) 

 in comparison with some other shells of this family. It is 

 about 2^in. long, lin. high, and Jin. thick, and of a yellowish- 

 brown colour, the lines of growth being well marked. The 

 umbones are towards the anterior extremity, and turn a little 

 inwards. The anterior side slopes away towards the ventral 



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