32 MULTiVALVES AND BIVALVES. 



At the outset let us have a diagram to illustrate the few terms 

 we shall have to use. Here is one valve of a shell. At the top is the 

 umbo or beak, which in this instance is not in the middle of the 

 upper edge, but a little to the side to which it points. That side is 

 the anterior side. Beaks almost always point forwards, and the 

 animal's mouth is always at the fore end, the siphons, when they 

 are present, being at the posterior end. Roughly parallel to the 

 lower edge of the shell is the pallial line, which is the muscular edge 



Posterior pedal redact-on 



An her! or pedal refractor 



adductor 



Posterior &dducl-or 



Pedal prohracf-or 



LEFT VALVE OF Unio pictorum. 



of the so-called mantle which secretes the shell ; the sinus, or 

 indentation, caused by the muscles which retract the siphons, 

 breaks into the pallial line, and is always on the posterior side ; and 

 the longer the siphons the larger the mark. When the siphons are 

 not retractile, there is no sinus. Siphons are the muscular tubes 

 formed by the extension of the mantle, one of them being branchial 

 and inhalent, and the other exhalent : so that there is a steady 

 current of water down the branchial tube and up the other one. 



At each end of the pallial line is a scar ; these are the marks 

 made by the adductor muscles which close the shell. When there 

 is but one muscle it is always the posterior, and is consequently 

 nearer that side than the other. These are not the only scars in 

 every shell. In the third diagram we have one of the Unionidae, 

 in which it will be seen that above the posterior adductor, and 

 apparently in continuation of it, is the mark of the posterior pedal 

 retractor muscle which draws in the animal's foot. Adjoining the 

 anterior adductor is the corresponding anterior pedal retractor, and 

 below it is the pedal protractor, which protrudes the foot. 



The ligament opens the shell, and is, as a rule, behind the beaks, 

 the depression around it outside, when there is one, being the 

 lozenge, or escutcheon, the area in front of the beaks being the 

 lunule. When the hinge line is unusually long, the ligament extends 



