422 Mr. WHEWELL on the Classification 
OBLONG-PYRAMIDAL SYSTEM. 
Remarks. 
Class. Combinations. 
Example and Figure. 
Class IIT. 
Pr, P’r 
OntIg 
pPr, pP’r 
Pr, oP'’r o Pr, gP'r 
x’ y  |M.VIII.2.) or oP’r, pPr 
This is not to be con- 
founded with square prisms 
0Q, ~Q, and 0Q, oQr. 
oPr, ofr OP, © Pr, a Pr 
a y 
Tue Hemihedral forms in this system, are those which contain the 
faces on one side of the axis only, at each summit. They are designated, 
as before, by the prefix h. 
There are also Tetartohedral forms, which exhibit only one out of 
the four faces given by each law. 
There are also forms in which the axis of the oblong pyramid is 
oblique, and these require other modes of investigation. ; 
The OcraHEpRAt System, including the combinations of figures sym- 
metrical in all directions (cubes, octahedrons, dodecahedrons, icositessera- 
hedrons, &c.), requires to be treated in a manner somewhat different from 
the preceding, and will not be here considered. 
The application of the preceding classification will be easily seen. 
In any proposed form, the relations of the faces and edges, being compared 
with the preceding descriptions, and with the figures here given, or with 
those referred to in Professor Mohs’s Mineralogy, will shew to what Class 
and Number the combinations of its faces may be referred. This will 
give us data for discovering the symbols of its faces, and, by connecting 
such data, we are to obtain the laws of the derivation. 
