220 3Ir. Gray's Conchological Observations. 



a similar purpose, that of supporting the head without muscular 

 power when the head is dependent for the purpose of grazing. 

 This ligament is placed on the inner side, close to or partly 

 attached to the adductor muscle, as may be observed by cutting 

 the body usually so called across, when the two substances will 

 be most distinctly visible, the one muscular, and the other 

 eminently fibrous and pearly. Now this ligament allows the 

 valves to separate to a certain distance, and no further. The 

 natural position of the valves, when at rest, is separate from 

 one another, as far as this ligament permits them to open, as 

 every body Avho has observed the habit of the animals will allow ; 

 but when any thing touches the animal, the adductor ~'jscle 

 is immediately put in action and the shells are closed ; but 

 they can only remain closed, till the muscles are tired. That 

 the valves are kept from separating beyond these limits by this 

 elastic ligament is proved by the circumstance, that when the 

 animal is dead, and the muscle is in a nearly decayed state, the 

 valves are kept at this distance from each other, or when the 

 muscular part is cut through, the valves do not exceed these limits ; 

 but when the adductor ligament and the muscle are both cut 

 through, the valves spring open to nearly as wide as they can 

 witliout breaking the true ligament, unless where they are stopped 

 by processes on the outside of the shell, as the beaks or the plates 

 which cover the ligament, as in the Cythereae. 



This admirable structure was first pointed out by my friend 

 Dr. Leach, who read a paper ou it, and demonstrated the fact, 

 before the Royal Academy of Paris; much to their amusement, as 

 being the first Englishman who had read a paper at their Academy, 

 and I need not add, with his vivacity, that he did not disgrace his 

 country. This discovery was noticed in the Bulletin of Science, 

 for 181 8 ; and Dr. Leach gave a short notice of it in the Annals of 

 Philosophy for 1820, p. 111. but it appears that his view has not 

 been clearly understood either by the English or the Continental 

 Naturalists. 



