49 CRYSTALLOGRAPHY. 
that is, in the clinodiagonal zone; and this is a consequence of 
the right angle which axis 6 makes with both axis c and axis a. 
The plane 2-2 is called the orthopinacoid, it being parallel to the 
orthodiagonal; and the plane 7-2, the clinopmacoid, it being 
parallel to the clinodiagonal. 
Vertical rhombic prisms have the same relations to the lateral 
axes as in the trimetric system. Domes, or horizontal rhombic 
prisms, occur in the orthodiagonal zone, because the vertical 
axis c and the o1thodiagonal 6 make right angles with one 
another. In fig. 6 the planes 1-2, 2-2 belong to two such 
domes. They are called andre. because parallel to the 
clinodiagonal. 
In the clinodiagonal zone, on the contrary, the planes above 
and below the basal plane differ, as already stated, and hence 
there can be no orthodomes; they are hemiorthodomes. ‘Thus, 
in fig. 6, 4-2, 1-2 are planes of hemiorthodomes above 2-2, and 
—4-iis a plane of another of different angle below 74. The 
plane, and its diagonally opposite, make the hemiorthodome. 
The octahedral planes above the plane of the lateral axes also 
differ from those below. Thus, in figs. 5 and 6, the planes 1, 1 
are, in their inclinations, different planes from the planes —1, 
—1;soinallcases. Thus there can be no monoclinic octahedrons 
—only henuoctahedrons. An oblique octahedron is made up of 
two sets of planes; that is, planes of two hemioctahedrons. 
Such an octahedron may be modelled and figured, but it will 
consist of two sets of planes: one set including the two above 
the basal section in front and their diagonally opposites behind 

(fig. 9), and the other set including the two below the basal sec- 
tion and their diagonally opposites (fig. 10). 
A hemioctahedron, since it consists of only four planes, is 
really an obliquely placed rhombic prism, and very frequently 
they are so lengthened as to be actual prisms. 
