168 



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 of 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 M. Walch, 

 consider as anapophysis, 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 ossiculae of this animal, which I have considered as the 

 ribs, was 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 membraneous flocculae 

 were very distinctly observed, depending from various parts of the 

 fossil. 



The fossil, PlateXIII. Fig. 10, known inthis 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 disfcsare, 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 



