Mr. J. S. Miller on Belemniics. 51 



probable that in the progress of crystalhzation these pellicles would have been 

 disarranged and obliterated. 



Several circumstances induce me to believe that the last or outer chamber 

 of the conical shell could at no time have possessed any great depth; for the 

 thin parietes of the conical shell could have acquired but little additional 

 strength from the extension of the edges of the last septum ; and had they 

 therefore projected far beyond it^ so frail a fabric would have been liable to 

 be squeezed in by the lateral pressure of the water. Indeed, a section of a 

 perfect specimen imbedded in its matrix^ in my possession, in which the ter- 

 mination of the parietes of the upper chamber is distinctly to be traced run- 

 ning to a sharp edge, confirms this fact, — that the outer chamber was like the 

 rest, very shallow. 



The chambered shell appears also from this and several other specimens to 

 have considerably extended above and beyond the upper extremity of the 

 guard. Hence we may conclude that the casts of large chambers often found 

 separate, do not indicate that the guard was of equal size. From the shell 

 being frequently traversed by numerous septa, it acquired a considerable de- 

 gree of strength and a power of resistance to the pressure of the surrounding 

 water, whilst it aided the locomotive power of the animal by its buoyancy. 



V. Particular Remarks on the Structure of the external Guard or Sheath of 



the Belemnite. 



The form of the guard or sheath varies in different species. In some it is 

 fusiform, in others conical ; in some slightly elongated, in others it is of con- 

 siderable length ; in some it has a round, in others an elliptical circumference. 

 Its termination is either acutely pointed, or conical ; either compressed, or 

 mamillated ; and sometimes the apex is slightly aduncated. Some species 

 have one, and others two or three longitudinal grooves. The surface of some 

 is smooth, in others it is slightly granulated ; and on some species we may 

 trace impressions which evidently betray marks of muscles which have clasped 

 or encompassed this part of the Belemnite, and show the impressions of branch- 

 ing vessels. 



Some species are semitransparent, others are opaque : in the former we 

 may observe that the guard is formed of two or three nearly equal longitudi- 

 nal portions, and along the line marking their adhesion the Belemnite easily 

 separates by a slight blow with a hammer, or by being heated and afterwards 

 moistened with cold water. By the same means a similar organization may 

 be discovered in the opaque ones. The edges of these longitudinal portions 

 are irregularly waved, and adhere closely together. 



h2 



