54 
Troost on the Pyroxene. 
edges replaced by two faces with an inclination of 133*^ 51' 
to the primitive plane. 
3. Perioctsedral P. * An eight sided prism with oblique 
base. Here the sharp edges are replaced by two planes, 
forming, with the plane of the primitive prism, angles of 
136° 9'. 
It must be here remarked, that it often happens that the 
pyroxene having the eight sided prism for form, offers a 
four sided prism nearly rectangular, having the alternate 
four sides merely linear. Sometimes even these are want- 
ing entirely — which, nevertheless, has never been observed 
in the volcanic variety. 
4. Equivalent P.t A prism with twelve faces with 
oblique bases. The former form augmented by four faces 
placed to the right and left of the two secondary faces of 
the Perihexsedral, having angles with the primitive planes 
of 152° 59'. 
5. Bisiinitary P. % The variety Perihexsedral, with a 
diedral summit. 
6. Dihexsedral P. || The former having the terminal 
edge of the pyramid replaced by a single face. 
7. Sexoctonal P. § The same form with an eight sided 
prism. 
8. Triunitary P. IF The same as Bisunitary^ having an 
eight sided prism. 
* Mem. Mus. 1, p. 284, fig. 26. 
•j- Annal. Mus. vol. ix. et Jour. Min. vol. xxiii. p. 152, pi. 3. 
\ Traite 3, p. 84, fig. 140. 
!| Mem. Mus. 1, p. 283, fig. 26. 
§ Traite 3, p. 84. t Traite, fig. 141. 
