80 



Shell ; generally thick and solid, with the hinge variously composed 

 of more or less solid cardinal and lateral teeth, sometimes groove- 

 striated, either of which may be wanting ; outer surface of the 

 shell frequently tubercled or torinhled, sometimes, but very rarely, 

 spined ; inner surface silver-pearly, sometimes purple or pink ; 

 ligament external. 



In some of the canals and rivers of England are found, somewhat spa- 

 ringly, three kinds of toothed bivalves, called respectively the Painter's 

 Mussel, Unio pictorum, the Pearl Mussel, Unio margaritifera* and the 

 Swollen Mussel, Unio tumidus; but these are merely stragglers of a type 

 whose centre of creation is in the great rivers of North America. The 

 Ohio, Mississippi, and other rapids of the States, swarm with individuals of 

 upwards of 330 species ; in South America some fifty species have been 

 collected, and the genus is represented in Asia, chiefly in India and Siam, 

 by about the same number; Europe, including Britain, has only ten spe- 

 cies, Africa eight, and New Holland three. 



The animal of Unio, and, indeed, of the family generally, presents a 

 structure intermediate between that of the Oysters in which the mantle- 

 lobes are entirely free, and that of the Myas in which the lobes are united, 

 and have two tubular orifices behind for imbibing and rejecting. The 

 mantle-lobes of Unio, Anodonta, and the rest of the Naiades, are open in 

 front, and have a broad tongue-like foot exserted between them, and closed 

 behind, forming orifices which are, as it were, the rudiments of siphons, 

 the upper one being small and simple, whilst the lower is larger and 

 fringed with a beard of short jagged cirrhi. 



The shells of Unio have been imported from North America in such 

 quantities, packed frequently in casks, that the collector sometimes tires of 

 the genus ; but they are of the highest interest, often beautifully sculptured 

 without, and richly iridescent with purple or silvery pearl within. The 

 pearl-producing Unios are chiefly those which inhabit rocky and turbulent 

 streams. In Britain it is from the Unio margaritifera of North Wales 

 that pearls are mostly extracted. It is in the roaring Conway, which flows 

 from the mountainous districts of the Principality, and is met in its rugged 

 course at every turn with boulders and splinters of rock, that the Unio 

 margaritifera is most disturbed in its calcifying operations, and sputters its 

 nacre in clumsy heaps of nodules, or in the superfluous pea-like balls 

 which go to decorate the necklace or tiara. In the rapids of Cumberland, 



* " The shells of this genus have frequently been used by painters for containing their colours, 

 and some of the species furnish pearls. The animals are not eaten in our country, but in the 

 south of Europe — where everything in the shape of shell-fish is devoured with an avidity which 

 defies starvation as long as rivers and seas yield mollusca in their present abundance — they are 

 cooked for food, either roasted in their shells, and drenched with oil, or covered with bread 

 crumbs and scalloped." — Forbes and Hanley, Brit. Moll., vol. ii. p. 139. 



