of Crystalline Combinations. 413 
RHOMBOHEDRAL SYSTEM. 
Relations of Edges, &c. 
In this class the plagihedral faces are turned opposite ways at the two apices: that is, 
twisted to the right at one apex, and to the left at the other. 
The plagihedral faces appear on the alternate angles, and alternately at the two 
apices. 
The plagihedral faces appear on the alternate angles, and simultaneously at the two 
apices. 
The plagihedral faces appear on all the angles, and at each apex. 
Their Edges of Combination are parallel to each other and to the lateral edges of the 
pyramids pR, pR’. 
SQUARE-PYRAMIDAL COMBINATIONS, 
Relations of Edges, §c. 
The forms are in a diagonal position. 
The preceding form in these combinations truncates the edges of the succeeding. 
The Edges of Combination are parallel to each other and to the terminal edges of the 
more acute pyramid. 
sb eee e tees acces esessesvesese also to the lines bisecting the lateral edges of 
the more obtuse pyramid. 
The forms are in a diagonal position. 
The preceding form replaces the summit of the succeeding. 
E. C. horizontal (A). 
E. C. horizontal (A). 
A face perpendicular to the axis. 
A regular four-sided prism, terminated by a pyramid in a parallel position. 
E. C. horizontal (A). 
