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. Bisunitary ~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. IT The same as Bisunitary, having an 

 eight sided prism. 



* Mem. Mus. 1, p. 284, fig. 26. 



\ 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. 1 Traite, fig. 141 



