1911 - 12 .] 
The Geometry of Twin Crystals. 
423 
A list of the symbols of the crystallographic classes is given in the 
accompanying table, with the usual names of the systems and classes, 
the name of a substance (where known) crystallising in each class, and the 
type symbols of the lines that may occur in it. It will be found convenient 
to refer to the classes by their symbols. 
Class 
Symbol. 
System. 
Class. 
Example. 
IUu 
triclinic 
asymmetric 
calcium thiosulphate 
lUc 
5 ? 
pinakoidal 
plagioclase 
II Uk 
monoclinic 
domatic 
clinohedrite 
II Uu 
33 
sphenoidal 
cane sugar 
II Uc 
>> 
prismatic 
augite 
II Bn 
orthorhombic 
pyramidal 
hemimorphite 
II Uh 
bisphenoidal 
epsomite 
II Be 
bipyramidal 
staurolite 
IV Uk 
tetragonal 
bisphenoidal 
unknown 
IY Bk 
33 
scalenohedral 
chalcopyrite 
IV Uu 
pyramidal 
wulfenite 
IY Bu 
yy 
/ ditetragonal \ 
\ pyramidal / 
trapezohedral 
succin-iodimide 
IY Uh 
yy 
strychnine sulphate 
IV Uc 
•>y 
bipyramidal 
scheelite 
IY Be 
j ditetragonal \ 
/ bipyramidal / 
rutile 
III Uu 
III Bu 
/ rhombohedral 
1 or trigonal 
J trigonal 1 
) pyramidal J 
/ ditrigonal 1 
sodium periodate 
>> 
\ pyramidal J 
pyrargyrite 
III Uh 
yy 
trapezohedral 
a quartz 
III Uc 
yy 
rhombohedral 
phenakite 
III Be 
yy 
scalenohedral 
calcite 
VI Uk 
hexagonal 
/ trigonal \ 
\ bipyramidal J 
unknown 
VI Bk 
yy 
J ditrigonal \ 
\ bipyramidal J 
benitoite 
YI Uu 
5 ? 
pyramidal 
nepheline 
YI Bu 
yy 
j dihexagonal \ 
\ pyramidal j 
iodyrite 
YI Uh 
yy 
trapezohedral 
B quartz 
YI Uc 
yy 
bipyramidal 
apatite 
YI Be 
33 
/dihexagonal \ 
\ bipyramidal / 
beryl 
f tetrahedral 1 
C Uu 
cubic 
1 pentagono- J- 
1 dodekahedralj 
sodium chlorate 
C Bu 
CUh 
yy 
f hexakis 1 
\ tetrahedral j 
pentagono- ) 
diamond 
yy 
] icositetra- V 
( hedral J 
cuprite 
CUc 
yy 
/ dyakis do- \ 
X dekahedral J 
pyrite 
C Be 
yy 
/ hexakis \ 
\ octahedral / 
galena 
Types of Line in each 
Class. 
lUu, 
lUc. 
lUu, lBu, 2Uk. 
IU 11 , lUh, 2Uu. 
lUc, lBc, 2Uc. 
f lUu, lBu, lUh, 2Bk, 
1 2Bu. 
lUu, lUh, 2Uh. 
lUc, lBc, 2 Be. 
lUu, lUh, 4Uk. 
( lUu, lBu, lUh, 2Bk, 
\ 2Uh, 4Bk. 
lUu, lUh, 4Uu. 
f lUu, lBu, lUh, 2Bk. 
\ 4Bu. 
lUu, lUh,2Uh, 4Uh. 
lUc, lBc, 4Uc. 
lUc, lBc, 2Bc, 4Bc. 
lUu, 3Un. 
lUn, lBu, 3Bu, 2Uk. 
lUu, lUh, 3Uh, 2Uu. 
lUc, 3Uc. 
lUc, lBc, 3Bc, 2Uc. 
lUu, iBu, 6Uk. 
/ lUu, lBu, lUh, 2Bk, 
\ 6Bk, 2Bu. 
lUu, lUh, 6Uu. 
/ lUu, IBu, lUh, 2Bk, 
1 6Bu. 
lUu, lUh, 2Uh, 6Uh. 
lUc, lBc, 6Uc. 
lUc, lBc, 2Bc 6Bc. 
lUu, 3Uu, lUh, 2XJh. 
j lUu, IBu, 3Bu, lUh, 
\ 2Bk, 4Bk. 
f lUu,lUh,3Uh,2Uh, 
{ 4Uh. 
lUc, 3Uc, lBc, 2Bc. 
/ lUc, lBc, 3Bc, 2Bc, 
\ 4Bc. 
