ITS MODIFICATIONS. 167 



Class d. Edges of prism replaced by tangent planes. 



The equal inclination of the planes d, upon the 

 adjacent primary lateral planes, distinguishes these 

 secondary forms from those of the right rectangular 

 prism. 



Class e. Edges of prism replaced by two planes* 

 The modifications of the terminal edges alone, or 

 the lateral edges alone, will tend to distinguish the 

 secondary crystals belonging to this class of prisms, 

 from those derived from the cube. 



A close resemblance may be remarked between the 

 primary and secondary forms of this class of prisms, 

 and those of the octahedron with a square base ; and 

 it is only from the cleavage that we are enabled to 

 decide which of these forms is to be regarded as the 

 primary, in reference to such secondary forms as are 

 common to both. 



ITS MODIFICATIONS. 



Primary form. A right rectangular prism. 



The individuals belonging to this class will differ 

 from each other in the comparative length of the 

 three adjacent edges C, B, and G. 



