218 Mr. Gray's Conchological Obsertalions. 



part; for he obseryes, that " If the ligament be external it is 

 extended when the shell is shut, and if the muscle, which keeps 

 the valve close, be relaxed, the mere elasticity of the ligament 

 opens them. If on the contrary, the ligament be iaternal, it is 

 compressed when the shell is shut, but on the muscle which 

 keeps it so relaxing, the elasticity of the compressed ligament 

 serves to open it." How far different is the fact ! 



That part which is generally considered as the ligament is al- 

 \^ays formed of two very distinct substances, one of which is 

 always external, is somewhat analogous to the ligaments which 

 connect the bones of Vertebrata, and is like them quite in- 

 elastic ; it appears to be of the same substance as the periostraca, 

 and is attached to the outer upper margin of the shell. In some 

 shells it is very thick and distinct, in others it is very thin and 

 scarcely visible. In the Pholadae it appears to be thin, and to have 

 the shelly plates imbedded in its substance; this substance I am 

 inclined to call the ligament. 



The other substance which has been confounded with the for- 

 mer, is somewhat similar to the curtilage of the Vertebrata ; it is 

 eminently elastic and formed of parallel series of condensed trans- 

 verse fibres, which are directed from the hinge of one valve io the 

 similar part of the other. This substance is always situated 

 within the other or true ligament, sometimes quite close to it 

 and similar to it in form, in which case it is attached just below 

 the edge of the ligament, to a protuberance on the dorsal margin 

 of the shell ; this protuberance is very distinct in the ToUinadas 

 and is sometimes called the Callus; but, as my friend Mr. James 

 Sowerby observed, it may be better called, from its use, the Ful- 

 crum. At other times it is placed at a distance from the ligament, 

 in which case it is called internal, that is to say, it is enclosed in a 

 cavity amongst the teeth, or in one peculiar to itself, as in Mijoe. 

 This cavity from its triangular shape and concentric striae, occa- 

 sioned by the growth of the cartilage taking place at the internal 

 margin, and thus increasing in breadth and thickness so as to form a 

 triangular wedge, has been called by Lister, the Cochleate or spoon- 

 shaped cavifj/. This body I call for the present the cartilage. 



Now when the valves are brought together by the action of the 



