316 PROCEEDINGS OF THE AMERICAN ACADEMY. 



signed to crystals from Andreasberg by Naumann 77 and Hausmann. 78 

 Irby 79 in commenting on this form throws some doubt on the correct- 

 ness of the determinations and points out the difficulties in the way of 

 distinguishing it from (6 -5 -IT • 1) and (7-6-13- 1) both well established 

 poles on either side of it. Sansoni 80 ascribes the form to Hausmann 

 without comment. The form should be classed as doubtful. 



w: (15- 13-28-2) = (15-0- 13). This scalenohedron was observed 

 by Stober 81 on crystals from Framont. On the combination figured 

 it takes the place of the much commoner and more rational form U : 

 (5491) = (504) shown in the three preceding figures (Figs. 9, 10 and 

 11). 



It is possible that the form in question should be referred to (7 • 6 • 

 13-1), a well established scalenohedron which lies 24|' from (15* 13 • 

 28-2) or to the hypothetical indices (8-7-15-1) which lies even closer 

 in zone. 



i : (17- 15-32-2) : (17-0-15). This scalenohedron was observed by 

 Sansoni 82 on crystals from Andreasberg. In spite of the close agree- 

 ment between measured and calculated angles recorded by Sansoni, 

 it seems doubtful that a scalenohedron should fall in this position in 

 the harmonic series. 



k: (21-9-40-2) = (21-0-19). This scalenohedron was observed 

 by Sansoni 83 on crystals from Arendal, Norway. The number of 

 scalenohedrons of the principal zone present on the combination de- 

 scribed by Sansoni suggests that the form under consideration may 

 possibly be vicinal. It should be regarded as rather doubtful. 



Hypothetical Scalenohedrons. Normal series N4 suggests the possi- 

 bility of a hypothetical scalenohedron which would reduce to 2 in the 

 series. Such a scalenohedron would have the indices (8-7-15-1) = 

 (807) and would fall between (15 -13-28-2) and (17-15-32-2). In the 

 opinion of the writer the angular difference between the hypothetical 

 form and the two above mentioned lies within the limit of fairly good 

 observation. The form should be retained as hypothetical. 



77 Naumann, C. T., Pogg. Ann., 14, 235 (1828). 



78 Hausmann, J. F. L., Handbuch der Mineralogie, 2 (1847). 



79 Irby, loc. cit. 



80 Sansoni, loc. cit. 



81 Stober, F., Abh. geol. Karte, Els-Lothr., 5, Fig. 12 (1892). 



82 Sansoni, loc. cit. 



83 Sansoni, F., Giorn. d. Min., 1, 2, p. 129 (1890). 



