164 THE FRESH-WATER MUSSEL [CH. VI 



elastic ligament posterior to the "umbones" serves to open 

 the shell ventrally by pulling the dorsal margins of the 

 valves together ; its action is opposed by the contraction 

 of the two adductor muscles. The "umbones" serve as 

 fulcra upon which the valves move as levers. Hence in 

 a dead specimen the valves always gape ventrally. 



The shell is composed of three layers ; an outer, or- 

 ganic, coloured periostracum, composed of "conchiolin" 

 secreted by the margin only of the mantle ; a middle, 

 calcified, white prismatic layer, forming in Anodonta 

 about half the thickness and also secreted by the mantle 

 margin ; and an inner lamellated calcareous mother-of- 

 pearl or nacreous layer. All round the margin of the 

 shell the periostracum projects and is slightly reflected 

 as a flexible sheet beyond the other layers. The nacreous 

 layer is iridescent owing to the optical effect produced by 

 the edge of the alternating lamellae of conchiolin and of 

 conchiolin charged with calcareous matter. This layer is 

 secreted by the entire surface of the mantle and through- 

 out life ; hence it steadily increases in thickness and is 

 thickest in the oldest portions of the shell. Pearls, which 

 are often found in Anodonta, are formed by this material 

 (vide infra p. 198). The ligament consists of an outer 

 layer continuous with the periostracum and an inner 

 which is striated radially; the latter is continuous with 

 the nacreous layer of the shell and at its margin the 

 radial fibres are connected with numerous muscle 

 fibres. 



In Anodonta, as the generic name implies, the valves 

 are destitute of hinge-teeth. In such lamellibranchs 



