XI.] THE FRESH- WATER MUSSEL. Ill 



yS. A curved ventral border, opposite' the dorsal. 



7. A wider anterior end. 



fi. A narrower posterior end. 



e. A right and left side. 



b. The mantle or pallium. 



a. A bilobed semitransparent membrane, one lobe 

 lining each valve of .the shell. 



yS. The continuity of the two lobes on the dorsal 

 side of the animal; their separation along 

 most of its ventral side, where each forms a 

 thick yellowish free border. 



7. The union of the two pallial lobes, for a short 

 distance, towards the posterior part of their 

 ventral border. 



5. The rudimentary dorsal and ventral siphons, 

 separated from one another at the point of 

 union 7. and each marked out by a part of 

 the mantle-edge covered by short hair-like 

 processes : the dorsal siphon completely closed 

 below and forming a narrow oval slit; the 

 ventral siphon open below and continuous with 

 the cleft between the ventral edges of the 

 mantle-lobes. 



e. 



The branchial or pallial chamber: turn back 

 the ventral edge of that mantle-lobe from which 

 the shell has been removed: a chamber is thus 

 exposed into which the ventral siphon and 

 the cleft continuous with it, lead. 



