828 PELECYPODA. 



valve.3 being held together and, in closing, approximated by the con- 

 traction of the adductor muscles, the preservation of their precise 

 apposition, marginally, is due to a rotatory motion, exerted along the 

 axis of the ligament, which pulls the attached edges of the ligament 

 nearer to each other and exerts a strain on its cylindrical exterior. 

 This operation, with a thin ligament, involves a tensile strain on the 

 whole cylinder ; with a thick ligament, the external layers are strained 

 and the internal layers compressed, so that to the tensile elasticity 

 of the external layers is added the compressional elasticity of the in- 

 ternal portion. The result of the differing stiains to which the several 

 layers of the ligament are subjected brings about a difference of struc- 

 ture, and wherever 'the ligament becomes deep-seated there is a 

 tendency for the respective parts to separate along the line where 

 the two sets of strains approximate. We then have two elastic 

 bodies operating reciprocally in opposite directions the outer, or 

 ligament proper, tending to pull the valves open to a certain distance, 

 corresponding to its range oT tensional elasticity ; and the other, or 

 resilium (cartilage or internal ligament), tending to push them open 

 to an extent corresponding to its range of expansion. As may be 

 seen by examining the unbroken resilium (as in Mactra), this organ 

 in such cases has something of an hour-glass shape, the ends which 

 fit into the "cartilage-pit," ' fossette," or resih'/ci'ft leing more ex- 

 panded then the centre between them. The deposit of lime in the 

 form of an accessory shelly piece, usually termed the o-sxict/lum or 

 lithodesma, serves for the reinforcement of the resilium. 



For the type of ligament which extends on both sides of the beak 

 Neumayr adopts the designation ainplnJHic. and for the more per- 

 fected type which has been withdrawn wholly behind the beaks he 

 employs the term opisthodetic. (rl>/ct>nt("ris offers a conspicuous type 

 of the amphidetic ligament : Tallinn and Venus exemplify the opis- 

 thodetic arrangement. 



The most perfected type of ligament is that which may be com- 

 pared to a cylinder split on one side, and attached by the severed 

 edges, one edge to each valve. This type is known as parivincular 

 (Tellina, Cardium) : its long axis corresponds with the axis of motion 

 or vertical plane between the valves, and its position is usually opis- 

 thodetic. Another form is like a more or less flattened cord extending 

 from one umbo to the other (Spondylus, Lima), with its long axis 

 transverse to the plane of the valve -margins and the axis of motion. 

 This is called alivincular. It may be central or posterior to the beaks, 

 but, unless very short, is usually associated with an amphidetic area. 

 A third form consists of a reduplication of the alivincular type at 

 intervals upon the area (Perna, Area), either amphidetically or upon 

 the posterior limb of the cardinal margin. This is designated as 

 multivincular, and is developed out of the alivincular type. (Ball.) 



Hinge. The valves of Pelecypoda are generally articulated, below 

 the umbones. bv a hhxjc. which is furnished in the majority of cases 



