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NEW YORK STATE MUSEUM 



binary symmetry and there are moreover, two vertical planes 

 of symmetry (fig. 67, no. 4, 5). 



This symmetry admits of two new forms. 



Fig. 85 Fig. 86 



1 The tetragonal sphenoid (fig. 85, 86) is composed of four isos- 

 celes triangles which meet the horizontal axes at equal dis- 

 tances; they also intersect the vertical axis. Two sphenoids 

 are possible which include all the faces 

 of the pyramid of the first order of the 

 normal group. The form is analogous to 

 the tetrahedron of the isometric system. 

 2 The tetragonal scalenohedron (fig. 

 87) is composed of eight scalene triangles, 

 which intersect the horizontal axes un- 

 Fig. 87 equally. As with the sphenoid there are 



two complementary scalenohedrons possible for every different 

 ratio of the intercepts on the horizontal axes. Up to the present 



Fig. 88 Fig. 



time the scalenohedron has been found only in combination with 

 other crystal forms of this group. 



Some of the combinations in this group are shown in fig. 88 

 and 89, which illustrate crystals of chalcopyrite. 



