CH. VI] THE FRESH-WATER MUSSEL 163 



hinge-line, along which the two valves are united by a 

 strong elastic ligament, is straight and corresponds with 

 the dorsal surface of the animal. The outline of the rest 

 of the shell may be described as roughly oval, the long 

 curved margin being ventral, the blunter end anterior 

 and the more tapering posterior. 



The external surface is marked by concentric lines 

 parallel with the margin ; these " lines-of-growth " are in 

 sets which probably each represent the growth of one 

 year. Very few exact observations have been made as 

 to the rate of growth, nor are the artificial conditions 

 of an aquarium suitable for experiment. Buxbaum 1 has 

 however recorded a valuable instance : a specimen of 

 Anodonta cellensis, known to be not more than three 

 years old, measured 135 mm. in length, 75 mm. in breadth 

 and 45 mm. in thickness ; its shell exhibited two strongly 

 marked lines marking out three sets of more faintly 

 marked areas which may reasonably be supposed each 

 to represent the increase gained in a single year. The 

 umbo is the oldest part of the shell and the line nearest 

 to it (usually very faint) marks the margin of the original 

 shell of the young mussel. The curvature of the lines 

 of growth is not quite smooth, but at about the middle of 

 the ventral surface tends to become convex in the dorsal 

 direction : this irregularity, the cause of which will be 

 mentioned later (vide p. 193), is more marked in the earlier 

 lines of growth than the later, and is more evident in 

 some specimens than in others ; it is usually pronounced 

 in species of Unio and less conspicuous in Anodonta. The 



1 Zoolog. Garten, xxxi. 1890. 



112 



