338 PROCEEDINGS OF THE AMERICAN ACADEMY. 



stitution of (4 • 13 • 17 • 6) = (958) for it by Descloizeaux is a somewhat 

 arbitrary one. Descloizeaux' substitution seems to have been based 

 upon the fact that (4 -13-17 -6) marks the intersection of the zones 

 [3142 :0772] and [0552 :5051]. The assumed pole is at a consider- 

 able distance from (5 • 13 • 18 • 7) in both <p and p values, several observed 

 forms being nearer. (5 -13 -18 -7) is referable to normal series N4 

 and marks the intersection of the zones [2201 : 0441] and [0221 : 5161], 

 which in fact is a somewhat better zonal relation than that of Des- 

 cloizeaux' assumed form. The form is probable. 



SCALENOHEDRONS OF THE ZONE [0551 : 5051 : 0110] = 



[223 : 11-4-4 : 112]. 



Splitting the first portion of this zone at (3251) = (302) which 

 developed a knot in zone 4 and developing the fragment between 

 (302) and (11-4-4) we have: 



Form 



Symbol 

 v- 1 



In this development the form (15-5-20-4) = (13-2-7) stands in 

 normal series N4. 



5(15-5-20-4) = (13-2-7). This scalenohedron is credited by 

 Zippe 116 to Haidinger 117 on crystals from Derbyshire. The figure 

 published by Zippe presents no zonal relations which would help in 

 establishing the rank of probability of the form. Goldschmidt re- 

 gards it as uncertain. On a basis of general zonal relations it should 

 be ranked as fairly probable. 



In the zone fragment between (5051) = (11-4-4) and (5491) = 

 (504) the following forms have been recorded : 



Form 



( n-_ \_ 

 Ul-4-4 412 

 Symbol 1 2 



*- 1 1 



N 3 ••• 1 



116 Zippe, loc. cit., Fig. 5. 



117 Haidinger, W., Leipzig (1829). 



