44 J. D. Dana on the Homeomorphism 
prism, J, and not to those between the centres of its opposite lat- 
eral edges. In other words, these lines are not the erystallo- 
graphic axes of the Trimetric system, but what the author has 
called the crystallogenic axes. This is one reason alluded to on 
a preceding page for believing that the crystallogenic axes are not 
necessarily the same lines with the crystallographic. The latter 
are lines assumed for the convenience of calculation. 
If instead of the domes 1i in section I, the species had afforded 
Zi as common and dominant forms, and these were taken as the 
unit domes, then the unit octahedron, in place of the domes, 
would have the pyramidal angles near 109°, approaching those of 
the regular octahedron. Could we therefore assume this as the 
fundamental octahedron for the species, the derivation of the oc- 
tahedron from the regular octahedron would be a change in the 
lengths only of the axes, and not in their angles of intersection. 
But this assumption would do violence to the facts. Still in An- 
timony Glance, we have an example probably of this form and 
mode of derivation; the dominant form is an octahedron, with 
is evidently the occasion of the wide divergence. Yet in one — 
nised as species that belong to a specific system of ratios, rathet 
than to definite and identical dimensions. 
Andalusite, Staurotide, and Topaz, have this relation. ‘The 
forms of these species may be referred to a similar type; yet W@ 
cannot affirm that the axes have the near identity presented iD 
the table, rather than a multiple ratio of 1:2 in some of the 
axes; we only know that they pertain to a common series. 
Staurotide alone offers a choice between three uncertainties 
The occurring form is a prism of 129° 20’ and this is usually 
by he 
eo aine author in vol. ix, p. 407, 2nd Series, of this Journal, and afterwards # 
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