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hibiting appearances most agreeable to his opinion, this circumstance 
appears to be more difficultly ascertained. The figures are here strongly 
relieved from the surface on each side, these forming the chief thick- 
ness of the stone, so that whilst rubbing these down, if much care be 
not taken by frequent examination, the intervening space is soon 
passed through, and the figure on the other side becomes gradually 
displayed. To make this examination with most success, the asteria 
should be rubbed down obliquely ; and then, on that part where it is 
most rubbed down, the figure will be found first to disappear, and an 
opportunity will thus be given, by which the intervening substance 
between the two stars may be discovered. 
This circumstance appears, however, to be completely decided by 
the agency of the muriatic acid in the preceding experiment, since, 
upon the separation of the crenated membrane, on which had been 
deposited the earth, forming that which we may, with Mr. Walch, 
consider as an apophysis, or rather as the cartilaginous medium for ar- 
ticulation, the acid was allowed to continue its action : and the sur- 
face from which this apophysis had separated, being then examined, 
was found perfectly plain. 
One of the ossiculie of this animal, which I have considered as the 
ribs, Avas also exposed to the action of the muriatic acid in a very di- 
luted state. In this instance, as in the former, as the calcareous earth 
was removed by the muriatic acid, numerous membranous flocculte 
were very distinctly observed, depending from various parts of the 
fossil. 
The fossil Plate XIII. Fig. 10, known in this country by the ill-applied 
term of Screw-stone, and called by the French Vis Depressoir, is a cy- 
lindrical column, composed of lenticular disks marked with fine ra- 
diating striae, attached to each other at the centre. The edges of 
these disks are, of course, from the apposition of two convex surfaces, 
separated at some distance from each other; considerable difference, 
however, is observable, in this respect, in different specimens. The 
