6:2 TERMINOLOGY. . 76. 



6. All these varieties are met with in hexahedral Iron- 

 pyrites ; the first also in hexahedral Cobalt-pyrites. 



. 76, PENTAGONAT>ICOSITETRAHEDRONS. 



The Pentagonal-icositetrahedrons, Figs. 29. 30., 

 are contained under irregular pentagonal faces, all 

 the angles of which are different, but which possess 

 two pairs of equal sides. 



1. These forms contain three kinds of solid angles, two 

 of which consist of three, and one of four faces. Eight of 

 those formed by three faces are equiangular and mono- 

 grammic ; these are situated like the solid angles of the 

 hexahedron : the other twenty-four are unequiangular and 

 trigrammie ; the situation of these is similar to that of the 

 trigrammic solid angles in the tetrahedral pentagonal-do- 

 decahedron (, 67. !.) The s i x so ^ angles of four faces 

 are equiangular, monogrammie, and correspond to the solid 

 angles of the octahedron. 



2. There are three different kinds of edges ; the first 

 terminate in those solid angles which are produced by the 

 concurrence of three equal edges ; the second terminate in 

 the solid angles of four faces ; and the third join those solid 

 angles with each other, which do not consist of equal plane 

 angles. 



3. The sections are rhombohedral and pyramidal, as 

 also their corresponding axes. The pyramidal axes pass 

 through those solid angles which contain four, the rhom- 

 bohedral axes through those which contain three equal 

 plane angles. These forms possess no prismatic axes at all, 

 and agree with the tetrahedral pentagonal-dodecahedrons, 

 In this particular as well as in the occurring difference be- 

 tween right and left. 



4. The dimensions of the three varieties of these forms 

 are as follows ; 



