THE SWAN MUSSEL 



3" 



Shell and 

 Mantle. 



a thick middle prismatic layer impregnated with salts- of 

 lime, and an inner nacreous layer of mother- 

 of-pearl, which consists of thin calcareous 

 lamina?. Lines of growth parallel with its edge 

 mark the outside of the shell, centring upon a point about 

 a quarter of its length from the front end. This point is 

 known as the umbo and shows the position of the first shell 



B 



Fig. 220 — The Swan Mussel. 



A, The shell with the animal, from the right side ; B, the left valve of the shell, from 



within. 

 d.s., Dorsal siphon; /., foot; /., impressions of muscles; i.a.ad., of anterior 



adductor; i.a.r., of anterior retractor; i.p.ad., of posterior adductor; /./.?'., 



of posterior retractor ; i.pra.^ of protractor ; l.g-., lines of growth ; paL, pallial 



line ; ub., umbo ; v.s., ventral siphon. 



