214 AX INTRODUCTION TO THE STUDY OF FOSSILS 



tie lobe and unites it to the shell. The mark left on the shell 

 by this attached mantle edge is called the pallial line. 



2. Siphonal. This is a specialized part of the mantle muscle, 

 originating from the siphons and serving to draw them 

 into the shell. It spreads out fan- wise in proportion to the 

 length of the siphons, and its insertion on the valve interrupts 

 the evenly curved line of attachment of the mantle muscle. 

 Hence the pallial line is indented by the pallial siims. 



3. Adductor. These are two large cylindrical muscles which 

 pass transversely across the body and are attached at each end 

 to one of the valves ; their insertion leaves on each valve an 

 anterior and a posterior muscle impression. The contraction 

 of these muscles pulls the valves together and closes the shell : 

 a chitinous ligament, tough and elastic, unites the valves ex- 

 ternally. This, like a C-spring, exerts a constant tendency to 

 straighten and thus to cause the valves to open to a certain ex- 

 tent. Beyond that distance it tends to prevent the valves from 

 opening. When the adductor muscles, therefore, relax, the 

 valves yield to the action of the ligament and open. 



4. Foot muscles. The foot is drawn into the shell by the 

 anterior and posterior retractor muscles and is extruded by a 

 single protractor muscle which spreads fan-like over the whole 

 visceral mass and by the compression of which the foot is forced 

 out. The attachment of these muscles is marked by three 

 faint scars on each valve — that of the anterior retractor and 

 the protractor near the anterior adductor impression, and that 

 of the posterior retractor just dorsal to the posterior adductor. 

 The foot itself is likewise very muscular. 



That part of the mantle which is outside the pallial line is 

 free, is somewhat thickened and contains shell-building glands. 

 This series of glands around the edge of the mantle secretes a 

 somewhat viscous liquid containing carbonate of lime, a shell 

 material probably derived from the blood by the epithehal 

 cells of the mantle margin. This hardens at once into a layer 

 of shelly matter deposited around the edges of the shell aper- 



