of Crystalline Combinations. 41] 
RHOMBOHEDRAL SYSTEM. 
Relations of Edges, &c. 
The form is an isosceles six-sided pyramid. If a=edge of rhombohedron, 
cos. 8 = J (cos. a— 2). 
See III. 1. The form is an isosceles six-sided pyramid; its edges at the summit 
replaced by the faces of another six-sided pyramid (pRr), which are rhombs. 
See IV. The form is a scalene twelve-sided pyramid. 
E. C. are parallel to the lines joining the apices with the bisections of the lateral 
edges of each pyramid. 
This form is called a Dirhombohedron. 
eS 
In this class the plagihedral faces are turned the same way at both apices: that is, 
_ at both to the right, or at both to the left. 
The plagihedral faces appear on the alternate angles, and alternately at the two 
apices. 
Their edges of combination are parallel to each other and to the lateral edges of the 
pyramid pR (A). See IV. 1. 
The plagihedral faces appear on the alternate angles, and simultaneously at the two 
apices. 
Their edges of combination are parallel to each other: those at one apex to the lateral 
edges of the pyramid pR, and those at the other apex to the lateral edge of pR’. 
(SS SS rR A ER ere 
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