5 



black colour and a pearly lustre. The cartilage is always situated 

 within the ligament, sometimes in immediate contact, and forming 

 with it one and the same mass ; at other times placed at a distance, 

 in a triangular cavity amongst the teeth of the hinge." 



It will be evident by a few examinations, that this statement, which 

 is intended to convey a general idea of the structure of all kinds of 

 hinge ligaments, is not reconcileable to all examples, but only to some 

 particular instances. 



If we take a portion of the ligament of the oyster, cockle, Cardium 

 costatum or Tridachna Hippopus, we shall find the two structures 

 above referred to, the external exhibiting no marks of structure, whilst 

 the internal exhibits something remarkable. 



On placing a fragment of the internal portion of the ligament of 

 Tridachna, it will be seen to be composed of numerous fibres, each 

 about the y^yq-t h part of an inch in diameter, running parallel to each 

 other, and apparently crossed by others at right angles ; but on sub- 

 mitting minute portions to very high magnifying powers, no trace 

 appears to exist of these cross fibres, but each fibre appears to 

 be composed of a cylinder, so formed as to present more or less trans- 

 parent and greenish markings, at regular distances from each other ; 

 and the coinciding of these distances or spaces, as the fibres lie side 

 by side, appears to give a transversely striated appearance. Nothing, 

 however, like the disks in muscular fibre is evident, nor is there a 

 spiral arrangement of atoms, but the stria? here visible are apparently 

 produced by some such method as they are on the primary fasciculus 

 of muscle, and there can be no doubt that this arrangement is con- 

 tractile, otherwise the shell could never have its valves opened when 

 the ligament is behind the hinge. 



In examining one of these fibres more in detail, we find its length 

 to be uncertain, probably extending from one valve to the other. In 

 looking at it attentively, it appears to be of uniform diameter, and 

 from its tenuity it decomposes the light in its transit, but the light is 

 not decomposed uniformly, for there appear to be certain transparent 

 spaces separated by green-coloured spots, the green colour apparently 

 given by the decomposition of the light. 



When we look to the office of this substance, which it appears evi- 

 dent is to open the valves, and which can only be done by this ma- 

 terial contracting like muscular structure, for when the shell is closed 

 it is overstretched, and contracts from elasticity. We have here a 

 considerable analogy between the one and the other in office : a con- 

 siderable analogy also exists between their structure, both being in 



