TRIMETRIC, OR ORTHORHOMBIC SYSTEM. 37 
in two directions at right angles with one another. The 
lateral planes of either square prism are alike in Justre and 
markings. 
Ill. TRIMETRIC, on ORTHORHOMBIC SYSTEM. 
1, Descriptions of Forms.—The crystals under the trimetric 
system vary from rectangular to rhombic prisms and rhombic 
octahedrons, and include various combinations of such forms. 
Figs. | to 7 are a few of those of the species barite, and figs. 8 
to 10 of crystals of sulphur. 

BARITE. SULPHUR. 
Fig. 11 represents a rectangular prism (diametral prism), 
and fig. 12 a rhombic prism, each with the axes. The axes 
connect the centres cf the opposite planes in the former ; but 
in the latter the lateral axes connect the centres of the oppo- 
site edges. Of the two lateral axes the longer is called the 
macrodiagonal, and the shorter the brachydiayonal. The verti- 
eal section containing the former is the macrodiagonal section 
aud that containing the latter, the brachydiagonal section. 
In the rectangular prism, fig. 11, only opposite planes are alike, 
because the three axes are unequal. Of these planes, that oppo- 
site to the larger lateral axis is called the macropinacoid, and 
that opposite the shorter the brachypinacoid (from the Greek for 
long and short, and a word signifying board or table). Hach 
