160 WALKER-ARNOTT, ON ARACHNOIDISCUS. 
Mr. Topping, and others in London, first observed these 
disks in Ichaboe guano. This guano was discovered in 1843, 
and was nearly all removed in the course of 1844.* Although 
guano was long known on the west coast of South America, 
I do not find that it was noticed for its diatoms until after 
that of Ichaboe was examined. 
Ehrenberg describes this genus under the name of Hemi- 
ptychus ornatus: when and where that name was given I have 
not traced, but believe it is in the ‘ Berl. Acad. Trans.’ for 
1848 or 1849. The description given in Pritchard’s ‘ Infus.,’ 
2d. ed., p. 382, shows that it is the form with transverse 
cost, and it is said to have occurred in “ Patagonian 
guano.” But here let me state that there is great difficulty 
in tracing the origin of guanos, not only from their being 
adulterated or mixed by the guano merchants, but by the 
preparers of objects for the microscope mixing what they got 
from different ships, under the impression that they were 
brought from the same place. Thus, some years ago, I pur- 
chased a slide of Diatoms from guano understood to be from 
Africa (Ichaboe); this contains the usual blue dises of that 
guano, but besides these is a valve (broken by pressing down 
the cover) of Hupodiscus (Aulacodiscus) formosus,+ which is 
peculiar to Bolivian guano. Here some guano from Arica had 
been mixed with that from Africa, the similarity of name pro- 
bably leading to the supposition that the two samples had come 
from the same locality. In Ehrenberg’s ‘ Mikrog.,’ tab. 35, 
he gives a representation of “ Peruvian” guano. In the 
description of the plate, however, it is stated to be from 
Arica, which is in Bolivia or Upper Peru, not in Peru as now 
limited. In the same work he exhibits the diatoms of two 
samples of guano from Saldanha Bay. The sample A. ap- 
pears correctly designed; but in B. all the species noticed 
(Endyctia oceanica, &c.) are so copious in Peruvian guano 
(called also Callao or Chinca), that I have no hesitation in 
saying that Ehrenberg must have misplaced the labels of the 
samples. 
I therefore doubt if the Hemiptychus ornatus was derived 
from Patagonian guano; but as the same form does occur in 
Californian guano, I dare not say that Ehrenberg’s was not 
from South America. 
* Whrenberg’s earliest notice of guano diatoms was in 1844, and his 
sample appears to have been obtained from London, and probably was 
derived from Ichaboe. 
+ Lu. (A.) formosus cupulis quatuor submarginalibus oblique mammeefor- 
mibus apice papillo instructis, granulis in 1-1000 parte septem (sive in 
1-100” parte sex) subaqualibus. — 
