58 THE BOOK OF NATURE STUDY 



three of these belong to the genus Unio, the other to the genus 

 Anodonta. These generic names happily indicate a feature by 

 which the animals can be distinguished. The Unios have strongly 

 marked hinge-teeth (union) uniting the two valves of the shell, 

 whereas Anodonta is without these structures (Greek, an devoid 

 of, odo ntes = teeth). A further distinction lies in the fact that 

 all Unios have a very thick, but Anodonta a thin shell. 



There are several varieties of Anodonta, differing from one 

 another in the shape of the shell and other slight features, but 

 they are now regarded as all belonging to one species, namely, Ano- 

 donta cygnea, the swan-mussel. It is to this species that the 

 present account is confined ; though Unio is so like in all main 

 characters that if it be used in practical work no difficulty will 

 arise in consequence of slight differences. 



The shell of the swan-mussel is roughly oval in shape, from 

 4 to 6 inches long, 2 to 3 broad from dorsal to ventral side, 

 and from I to 2j thick from right to left. In colour 

 it is greenish brown, though often dotted with white patches 

 owing to removal of the coloured surface layer. The two valves 

 lie on the right and left of the body, and resemble each other as 

 a reflected image in a mirror resembles the real object. The 

 anterior end of the shell is more rounded and blunter than the 

 posterior. Along the straight dorsal side the valves are held 

 together by a strong elastic ligament ; but elsewhere and along 

 the gently curved ventral margin they are free from each other. 

 The concentric lines running parallel with the margin and marking 

 the outside surface are known as " lines of growth." They in- 

 dicate the positions successively occupied by the margin of the 

 shell during its growth, and are divisible into sets by more pro- 

 nounced lines which probably represent periods of rest. The 

 interval between any two of the stronger lines is the increase 

 effected during one year. When the shell was very small the 

 lines of growth were close together, and there is thus marked out 

 on the dorsal region of each valve a sort of boss or " umbo." This 

 is the oldest part of the shell, and the thickest. The elastic liga- 

 ment tends to pull the dorsal edges of the valves together, each 

 umbo serving as a fulcrum upon which the other valve turns. 

 Thus in the resting position the valves gape apart ventrally, 



