58 TERMINOLOGY. . 71. 



3. The sections and axes are as in the tetrahedron. The 

 prismatic axes pass through the solid angles of four faces. 



4. There are three varieties, of the following dimensions: 



5. The third variety of this form has been observed in 

 hexahedral Boracite ; the other two depend upon the first 

 and second variety of the tetraconta-octahedron, . 77. 133. 



. 71. HEXAHEDRAL TRIGONAL-ICOSITETRAHE- 

 DRONS. 



The faces of the hexahedral Trigonal-icositetrahe- 

 drons, Fig. 32., are isosceles triangles. 



1. The solid angles consist either of four or of six faces, 

 and are all equiangular. The first, six in number, are mo- 

 nogrammic, and situated above the centres of the faces ; 

 the second, eight in number, and digrammic, are situated 

 like the solid angles of the hexahedron. 



2. Those edges of the form which correspond to the 

 edges of the hexahedron, join the angles of six faces with 

 each other; the others join the solid angles of six faces 

 with those of four faces. 



3. The sections and axes are as in the hexahedron. 



4. There are three varieties of these forms, whose dimen- 

 sions are the following ; 



a. b. A. B. 



1-79 31' 28". 50 14' 16". 157 22' 48" 133 48' 47". 



2. 83 37' 14". 48 11' 23". 143 7' 48" 143 7' 48". 



3- 86 58' 59". 46 30' 30|". 1<26 52' 12". 154 9' 29". 



