ORIGINAL COMMUNICATIONS. 
Norrs on Aracunorpiscus, Pxievrosigma, AMPHIPRORA, 
Eunotia, and Ampnora. By G. A. Watkrer-Arnott, 
{ih 
ARACHNOIDIscUS.—Since my observations on this genus 
were published in the last number of this Journal, at p. 160, 
my attention has been called to the ‘Annals and Mag. of 
Nat. Hist.,’ for 1848, vol. 1, p. 393, in which there is a 
translation of Ehrenberg’s paper on Hemipiychus. I am 
likewise informed by another correspondent that it was 
published in the ‘ Berlin Proceedings’ for 1848, p.7. I have 
not access to the original, but from what is said in the trans- 
lation, it is clear that Ehrenberg got his specimens from Mr. 
Topping, of London; and as it is generally well understood 
that the Arachnoidiscus was obtained by Mr. Topping from 
Ichaboe guano, the suspicion I threw out is verified. Whe- 
ther it was certainly from Patagonia, that the Danish vessel 
Waldemar (spoken of by Ehrenberg) brought its cargo of 
guano, I know not ; Ehrenberg understood this, and his infor- 
mation may have been correct, although doubts arise from 
the following considerations. From that guano Ehrenberg 
obtained the diatom which he has called Hntopyla australis, 
for which he quotes as a synonym his former Swriredla (?) 
australis ; now my impression, from studying attentively his 
generic character, is that the Entopyla australis is the same as 
my Eupleuria (or Gephyria) incurvata, which is from Ichaboe 
and Saldanha Bay guano; while his Sur. australis, from the 
Falkland Islands, and not, I believe, from guano at all, is 
probably my fu. ocellata. But it is almost impossible to 
determine this point without seeing perfect specimens. 
I may add, that I am now informed by M. De Brebisson, 
that the single frustule of Arachnoidiscus, which he detected on 
Sphacellaria olivacea, sent him by Myr. Ralfs, and which is the 
authority for that genus being British, belongs to A. ornatus,* 
and is therefore the same as from Ichaboe and the Cape. As 
the species of this genus, like other diatoms, are gregarious, the 
discovery of only one frustule seems to indicate some error 
about its title to a place in the catalogue of British genera. 
* Threnberg’s specific appellation was orvatus ; such (and not formosus) 
is the name which I obviously intended to give to the second Arachuoidiscus 
at page 162, about the middie. This readers will please to correct. 
VOL. VI. Q 
