compression, whilst the ligament behind the hinge, can only effect the 

 same end by contraction after being elongated after the closing of the 

 shell by the adductor muscles. 



This contrariety of action induced me to examine the structure of 

 the ligament in those shells that have come within my reach, and I 

 now propose to detail the results of my investigation. 



In directing the attention to the different bivalve shells, it can 

 readily be observed, without the assistance of the microscope, that 

 in those shells in which the ligament is in front of the hinge, its 

 structure appears horny and homogeneous, whilst, on the contrary, in 

 those shells where the ligament is beyond the hinge, the structure of 

 this organ appears to be composed of two distinct portions — an ex- 

 ternal, which is ligamentous and unyielding, and a central portion, in 

 which appears to reside the power of contraction. This latter, in the 

 dry state, appears friable and of a whitish colour, whilst the former 

 possesses a black colour and a vitreous fracture. 



Structure. — In examining the structure of the ligament placed be- 

 fore the hinge, it will be found that in certain species of Spondylus 

 and Pecten, this organ manifests no perceptible structure, being to all 

 appearance structureless, like " Indian rubber," whilst in other ex- 

 amples, as the pearl oyster, it is fibrous, and in the common mussel 

 {Mytilus edulis), a very singular structure is discernible. 



If a portion of the ligament taken from this animal be cut so thin 

 as to be transparent, it will appear under a high magnifying power to 

 be composed of a dense tissue, without any particular structure, in 

 which are certain small channels, or lacunas, filled with fluid ; from 

 this arrangement it would appear, that when the ligament is com- 

 pressed by the adductor muscles approximating the valves, the 

 presence of fluid in these lacuna; (by being incompressible) serves to 

 render more tense the ligamentous structure, and thereby increase its 

 elasticity. 



When the attention is directed towards the structure of the ligament 

 placed beyond or outside the hinge, it will be found to be composed, 

 as already stated, of two substances possessing a different organiza- 

 tion. This fact has been long known, for Dr. Roget, in his ' Bridge- 

 water Treatise,' vol. i. page 217, says, "This ligament is composed 

 of two kinds of texture, the one which is always external is strictly 

 ligamentous, that is, perfectly inelastic; the other has more of the pro- 

 perties of cartilage, being highly elastic, and formed of a parallel 

 series of condensed transverse fibres, directed from the hinge of one 

 valve to the similar part of the other, and having generally a deep 



