1356 
each other. Also in this case, therefore, the agreement between 
observed and expected symmetry appears to be beyond doubt. 
VI. Tetragonal-pyramidal Class. 
The only representative of this class at our disposal was Wu/fenite: 
PbMoO,; the results obtained with it are therefore reviewed here. 
The axial ratio of this mineral is: a:e=1:1,5777. The beautiful, 
orange-coloured crystals originated from Yuma Co in Arizona; 
later-on we investigated also yellow crystals of Mammoth Mine in 
the same country. 
The Röntgen-pattern parallel to (001) (d= 0,24 mm.) is reproduced 
in fig. 10 on Plate III, and as a stereographical projection in fig. 16 
on Plate IV. Apparently, perhaps also really, this image shows an 
analogous increase of the theoretical symmetry, as was found in the 
case of scheelite. The images parallel to (100) and (110) also, al- 
though too feeble to be reproduced here as proof, make the pre- 
sence of two planes of symmetry, perpendicular to each other, very 
probable. However, the innermost circles of spots show on the 
negative parallel to (OOI) a distribution of the intensities, as is 
indicated also in the corresponding stereographical projection, and 
which might prove that a lower symmetry yet manifests itself in 
this Ro6ntgen-image, although only in a very feeble degree. In con- 
nection with the mentioned symmetry of the images parallel to (100) 
and (110) however, this supposed lower symmetry may appear still 
rather doubtful *). And in that case here again the question must arise, 
what the cause can be of this increase of the expected symmetry ? 
Here too a possible explanation of the fact could only be 
based upon the supposition of a twinformation, which however in 
the case of wulfenite has been observed rarely or not at all. The 
not completely adjusted image of the erystal of Mammoth Mine, 
betrayed the pressure of planes of symmetry also. 
Of the tetragonal-bisphenoidical class, there are, as already men- 
tioned no representatives known with certainty. 
§ 3. If thus we review here the results obtained in this invest- 
igation, we can draw attention to the fact, that for six of the 
seven classes of the tetragonal system, a study of the degree of 
symmetry of the RÖNraer-diffraction-images was made; and that, 
with the exception of the cases of scheelite, and perhaps also of 
wulfenite, — which both show an abnormal increase of their degree 
1) The particular shape of the spots in this case too makes it doubtful, ifreally . 
no twin-structure plays a certain role here. 
