274 HINGE-TEETH AND OTHER PROCESSES chap. 



uncommon upon the ventral margin in certain genera (Fig. 190). 

 The mechanical effect of these continued riblets, when fitted 

 together on the opposing valves, would be to prevent the valves 

 sliding upon one another while closing, or after being closed. 

 Thus there would be a probability of their surviving, even after 

 the ribbing had disappeared from the surface of the shell, the 

 increased strength given by the hinge compensating for, and 

 making it possible to do without, the extra strength supplied by 

 the ribs. It is therefore possible that the teeth of the Nuculidae 

 and Arcadae, which have no distinction between cardinals and 

 laterals, represent a very ancient type, from which have been 

 evolved the various forms of hinge in which cardinals and laterals 

 are distinguished. Even in some forms of Arcadae (comp. Pec- 

 tunculus) we get a hint how the transverse teeth of the typical 

 Area may have become transformed into the longitudinal tooth 

 of the normal lateral.^ 



The developed hinge-teeth, then, ensure the opening of the 

 valves in one direction ; they also secure their accurate closure 

 upon one another in exactly the same plane. Exposed shells 

 and gaping siphons matter little to animals which are protected 

 by their burrowing propensities, but to those which live in 

 material which can be easily penetrated by their foes, it must be 

 of advantage to be able to buckle their armour absolutely tight. 

 The edentulous hinge of Anodonta is a degeneration from a 

 dentate type, which retains its teeth (in Unio^ etc.) when subject 

 to the jar of rapid streams, but tends to lose them in the stiller 

 waters of canals, lakes, and ponds. 



Other processes in the bivalve shell. — In Anatina each umbo 

 is fissured and strengthened on the inside by a kind of umbonal 

 plate which carries the ligament. Some forms of Liligna de- 

 velop a strong internal umbonal rib, which serves as a buttress 

 to strengthen the shell. In Pholas, the so-called falciform pro- 

 cess serves as a point of attachment for the muscles of the foot 

 and viscera. There is no ligament or hinge-teeth, the place of 

 the latter being taken by the anterior adductor muscle, which is 

 attached to the hinge-plate, the latter being reflected back into 

 the shell. 



In Septifer the anterior adductor muscle is carried on a 

 sort of shelf or myophore^ and in Cacullaea the posterior 

 1 W, H, Dall, Amer. Joiirn. Sc. xxxviii. p. 445 f. 



