188 MEMORANDA. 
of the sides of the incomplete annuli, forming, when in 
juxtaposition, the zigzag appearance which first strikes the 
attention of an observer directed to the Rhizosoleniz, is no 
bar to the accuracy of the proposed union, for it is only a 
more developed state of a similar condition met with in 
Striatella, as will be seen by reference to the figure of the 
latter in the ‘Synopsis B. D.,’ vol. 11, pl. xxxix. 
The advantages of burning Diatomacez on tale are hardly 
yet sufficiently known or appreciated. In the more delicate 
the use of this mode of investigation is especially apparent. 
Viewed dry in this way, markings which balsam obliterates, 
are then clear and sharp, but it is especially in the making 
out of difficult points of structure, as we have just seen, and 
the exact appreciation of minute differences between allied 
species, that its value will be most felt. 
Very pretty specimens of Diatoma, Grammatophora, and 
the other forms of Diatomacee in which the frustules sepa- 
rate and cohere at alternate angles, have been obtained by 
lightly burning specimens placed on the slide, over the flame 
of a lamp. The admirable medium of Mr. Farrants pro- 
mises, however, to render the preservation of such specimens 
less difficult and uncertain. It will also, I expect, enable us 
to mount permanently specimens in conjugation, which have 
hitherto baffled every attempt made with this view, and if so 
we may confidently hope, by the diffusion of such mountings, 
to awaken further interest in this intricate subject.—TUuFFEN 
West, 8, Hemblington Cottages, Queen’s Road, Dalston. 
On the History of Arachnoidiscus—In reply to your note 
of this date I can say little beyond the fact that, on March 
17th, 1847 (just eleven years last Wednesday), I read a short 
unscientific notice “On the occurrence of Arachnoidiscus 
on an edible fucus from Japan,” at the end of which, on a 
question put by our friend John Quekett, I said, “ From 
the circumstance of these discs presenting a reticulated 
appearance, similar to the webs of some species of spiders, I 
propose to call the genus Arachnoidiscus, and the species 
above mentioned Japonicus. My notice did not give a 
scientific description of the genus intelligible to any one, and 
therefore, as I have since learned, I have no claim to it. It 
might have been otherwise had any one kindly suggested to 
my greenness the right course to render my notice of a new 
thing perfect. 
Dr. Walker-Arnott lately wrote to me on the subject at 
the suggestion of Dr. Harvey, who knew what I had done, 
and to whom I was indebted for species from Mauritius. He 
