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cavity of the other valve : he also pointed out, on the inner side of the 

 back part of the larger valve, the remains of a hinge formed of teeth with 

 corresponding depressions, with an analagous surface on the posterior 

 edge of the smaller valv^. 



M. Guettard was of opinion, from the appearance yielded by the 

 inner surtace of the larger valve, that it had proceeded from thu labours 

 of some zoophyte. M. Walch acknowledges that this opinion derived 

 some support from the appearance of coralline texture ; since, on exa- 

 mination with a glass, of the two specimens possessed by him, a very 

 fine texture, formed by intersecting threads, appeared, and which was 

 similar to that which is seen on some of the Muestricht fossils, but which 

 he was unable to determine whether it was accidental or not. 



This very curious fossil having very much excited my attention, I was 

 happy in obtaining a very complete specimen, at the sale of the late 

 Mr. Forster's collection, by which I am enabled to place before you an 

 accurate representation of both the valves. Plate XVI. Fig. 15, is the 

 anterior part of the larger valve, in which the superior margin and the 

 hinge part is in good preservation : at a is a magnified representation of 

 the tooth in the middle of the posterior margin of this valve. 



Plate XI. Fig. 14, is the internal surface of ttfe flat semicircular valve. 

 To procure a display of the structure of the hinge, in this valve, was 

 exceedingly difficult; but by the alternate employment of the instru- 

 ment, and of the muriatic acid, this view of it was obtained. It is there 

 seen, that in this hinge there are three parts which are particularly 

 observable : a long beak-formed tooth, in the centre of the posterior 

 margin, answering to the tooth in the posterior margin of the other 

 valve, and a protuberance at each end, formed by four or five alternate 

 ridges and furrows. 



The lens discovers the reteporean appearance on the internal surface 

 of the large valve ; a part of which, thus magnified, is represented at b. 

 From the circumstance of its occurring in both valves, but particularly 



