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exactly closing up the 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 analogous 
surface on the posterior edge of the smaller valve. 
M. Guettard was of opinion, from the appearance yielded by the 
inner surface of the larger valve, that it had proceeded from the 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 Maestricht 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 XYI. 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 XYI. Fig. 14, is the internal surface of the 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 
instrument, 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 h. 
From the circumstance of its occurring in both valves, but particularly 
