BASEBAND ITS MODIFICATIONS. 147 



Class p. Lesser edges of the base replaced by two 

 planes. 



This species of octahedron has been adopted here 

 as a primary form, in conformity with the opinions 

 entertained by the Abbe Haliy of its belonging to 

 certain species of minerals. But it is probable that 

 the right rhombic prism is really the primary form, 

 of most, if not of all, those species, and that there is 

 not any mineral whose crystals are strictly referable 

 to this class of octahedrons. 



BASE, AND ITS MODIFICATIONS. 



Primary form. An octahedron with a rhombic base. 



It has been already observed that this figure is 

 drawn with the greater diagonal of the base horizontal. 

 Hence the primary planes meet at the edge B, at a 

 more acute angle than at the edge C. The edge B 

 will therefore be denominated an acute edge of the 



O 



pyramid, and the edge C, an obtuse edge of the pyra- 

 mid. The solid angle at E will be termed the acute 

 lateral solid angle, and that at I the obtuse lateral 

 solid angle. 



The individuals belonging to this class will differ 

 from each other in the inclinations of P on P' and 

 on P". 



T 2 



