56 Shell Life 



so that the figure shows it in its natural position, 

 with the concave side towards us. In the com- 

 plete shell the upper margin of this valve is 

 connected with its fellow by a kind of hinge 

 (ligament), resembling those primitive leather hinges 

 by which boys attach lids to boxes and doors to 

 rabbit-hutches. This shell hinge, however, is more 

 horny than leathery, and is, so to speak, fixed on the 

 stretch, and this explains why when a bivalve 

 moUusk dies its shell gapes open : the hinge-ligament 

 exerts a pulling action which tends to separate the 

 lower edges. 



Just beneath the beak inside each valve there is a 



little spoon-shaped projection containing a bulging 



bit of elastic cartilage, which also serves the 



purpose of forcing the valves apart. On 



either side of this cartilage is a row of low 



Hinge-teeth ^.^(^lofes Ukc the tcetli of a comb, and into 



(enlarged) ® 



the spaces between similar teeth on the 

 right valve fit; the lower margin of each valve is 

 also faintly marked in a like manner. The number 

 of the hinge-teeth varies in different species ; in the 

 present one there are about 15 teeth in front of the 

 beak and about 25 behind it. The object of all these 

 interlockino- teeth is to ensure the accurate closinof of 

 the valves Avhen they are brought together, and to 

 prevent any lateral movement that would enable an 

 enemy to destroy the hinge. But the mechanism by 

 which the two valves are closed and held tightly 

 together, in spite of the opening action of the hinge- 

 ligament, consists of a couple of bands of muscle, one 

 towards each end of the shell. On the pearly inner 

 surface of the valve we can see slight oval impres- 



