TWIN, OR COMPOUND ORYSTALS. 55 
hedral forms may consist of 2 pianes (the bases, and each pair 
of diametral planes), of 4 planes (rhombic prisms in the three 
axial directions), and of 8 planes (the rhombic octahedrons). 
(4) The forms may be divided into equal halves, symmetrical in 
planes, along each of the diametral] sections. 
4, Monoviintc System.— (1) No symmetrical groups of 
similar planes ever occur around the extremities of either axis. 
(2) The prisms are rhombic prisms, and these can occur paral- 
Jel only to the vertical axis and the clinodiagonal. (3) The 
planes occurring in vertical sections above and below the basal 
section, either in front or behind, are all unlike in inclination 
to that section, excepting the prismatic planes in the ortho- 
diagonal zone ; in other words, true prisms occur in no vertical 
section excepting the orthodiagonal. (4) Simple forms consist of 
2 planes (the bases, the diametral planes, and hemiorthodomes), 
of 4 planes (rhombic prisms in two directions and hemioctahe- 
drons). (4) The forms may be divided into equal and similar 
halves only along the. clinodiagonal section. No interfacial 
angle of 90° occurs except between the planes of the clinodiag- 
onal zone and the clinopinacoid. 
5. Trictinic System. — In triclinic crystals there are no 
groups of similar planes which include more than 2 planes, and 
hence the simple forms consist of 2 planes only. The forms 
are not divisible into halves having symmetrical planes. There 
are no interfacial angles of 90°. 
6. Hexaconat SysTEem. —Symnietrical groups of 3, 6, and 12 
similar planes may occur about the extremities of the vertical 
axis. (2) Prisms occur parallel to the vertical axis, and are 
either six or twelve-sided (3 in a hemihedral form) and equi- 
lateral. (3) Simple holohedral forms may consist of 2 planes 
(the basal), of 6 planes (hexagonal prism), of 12 planes (twelve- 
sided prisms and double six-sided pyramids), of 24 plancs 
(double twelve-sided pyramids). Simple rhombohedral forms 
may consist of 2 planes (the basal), of 6 planes (rhombohedrons), 
and of 12 planes (scalenohedrons). 
2. TWIN, on COMPOUND CRYSTALS. 
Compound crystals consist of two or more single crystals, 
uxited usually parallel to.an axial or diagonal section. A few 
are represented in the following figures. Fig. 1 represents a 
crystal of snow of not unfrequent occurrcace. As is evident 
