49° TRANSACTIONS OF THE WAGNER FREE 



connects them, allows them at a later stage to move freely upon it as an axis, 

 and is known as the Ligament. The ligament may therefore be regarded as a 

 fundamental character of the Pelecypoda and is universally present, though in 

 some cases as a mere degraded rudiment (Pholadacea), and, in the case of 

 Chlainydoconcha, where the valves are encysted in separate sacs covered by 

 epithelial tissue, the adductors have vanished and there is no hinge, neverthe- 

 less so strong is the influence of heredity that the ligament persists in a cavity 

 by itself, disconnected from the valves, as a well-developed horny mass with- 

 out function. The absence of a ligament in the Rudistce, if primitive, would 

 result in a perfectly unique mechanical relation of the valves and muscles, as I 

 have elsewhere pointed out (Am. Jour. Sci., 1889, p. 461), and, considered in 

 connection with their other characters, would effectively separate them from 

 the true Pelecypods as a distinct division of Mollusca. It is, however, proba- 

 bly only an extreme case of specialization. 



As the most important factor in the mechanism of the valves the ligament 

 has undoubtedly developed with the development of the class and its chief modi- 

 fications date from the earliest period of the development of the group. I have else- 

 where shown that the function of the original ligament was that of an external 

 link between the valves having the essential nature of a C-spring. That is, the 

 insertion of the ligament edges on the external cardinal margins, or, with de- 

 velopment, on thickened ridges, by which these margins are reinforced (the 

 so-called nymphse) to bear the strains to which it necessarily became subjected, 

 resulted in the following conditions : The valves being held together, and, in 

 closing, approximated by the force exerted through contraction of the adductor 

 muscles, the preservation of their precise apposition, marginally, is due to a 

 rotary motion, exerted along the line of folding (or 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, in- 

 volves a tensile strain on the whole cylinder ; with a thick ligament the exter- 

 nal layers are strained and the internal layers compressed, so that, to the ten- 

 sile elasticity of the external layers, is added the compressional elasticity of the 

 internal portion. The latter condition is the most common among modern 

 forms, though it is probable that the former was the original type. I have 

 also shown that the result of the differing strains to which the different layers 

 of the ligament are subjected brings about a difference of structure, and, when 

 the ligament becomes deep-seated for any reason, there is a tendency for the 

 respective parts to separate along the line where the two sets of strains ap- 

 proximate. We then have two elastic bodies, operating reciprocally in oppo- 

 site directions, the outer or ligament proper tending to pull the valves open to 

 a certain distance corresponding to its range of tensional elasticity ; and the 

 other, the " cartilage," * or resilium, tending to push them open, to an extent 



* Tlie so-called "cartilage," which is not a cartilage, and which is frequently spoken of as "ligament" 

 or " internal ligament," is in great need of a distinctive name, and I propose that of " resilium," which clearly 

 indicates its function. 



