914 . Mr. J. Miers on Aixtoxicum. 
perfectly. It was curious to observe how, by diluting the acid 
with water, the valves again became visible and distinct in out- 
line markings. By igniting this earth I produced a slight change 
in the index of refraction of the silica composing the valves, so 
that afterwards they were visible with tolerable distinctness in 
the same sample of sulphuric acid. 
Index of refraction of water ....sesesesees 1°336 
3 > sulphuric acid........ 1435 
- 7H Dintoms ‘i. is saad oon 1°435 
- a Quartz scccsrsoreer ike > teal 
Canada balsam ...... 1°548 
This table shows that the index of refraction of the Diatoms 
is about half-way between that of water and Canada balsam, 
thus explaining the fact that they appear about equally distinct 
in both of these media. 
XXV.—On Aixtoxicum and Bursinopetalum. 
By Joun Miers, F.R.S., F.L.S. &c. 
JEXTOXICUM. 
Tue position of this genus of the Flora Peruviana has not yet 
been satisfactorily established ; but the observations I have been 
enabled to make will probably throw some light on the subject. 
The only botanist who has suggested a place for it is Sir Wm. 
Hooker, who, in 1887, referred it to Huphorbiacee; but the 
structure of the fruit and seed, with other characters, militate 
against this conclusion. Notwithstanding the anomaly of some 
of its features, the weight of the evidence here collected together 
will, I think, show its close proximity to the Aquifoliacee. The 
distinguished botanist just mentioned, when he described its 
flowers as being dicecious, had only seen the male plant; they 
are, however, properly speaking, polygamo-dicecious: that is to 
say, in the male flowers the ovary, though present, is rudimentary 
and sterile; and in the female flower, although the stamens of 
the usual size exist, the anther-cells are abortive and deficient of - 
pollen: the two sexes are always in distinct plants, and in all 
these respects the genus resembles Prinos and Nemopanthes : 
so far, therefore, ztoxicum is quite reconcileable with the 
Aquifoliacee. Its only known species is a native of the central 
and southern provinces of Chile, where it forms an evergreen 
tree of some height, with oblong alternate leaves, covered with 
lepidote scales, Its flowers are small, in simple axillary racemes 
much shorter than the leaves, the whole inflorescence being 
densely covered with lepidote scales similar to those of the leaves. 
Each flower, while in bud, is closely invested by a membrana- 
