162 WALKER-ARNOTT, ON ARACHNOIDISCUS. 
in his plate 31. I can only explain this by supposing that 
Mr. Tuffen West, in making the drawing had employed a 
specimen of the true Japan form, perhaps from Mr. Deane; 
while Smith had derived his generic character solely from 
African specimens, aided, perhaps, by Mr. Shadbolt’s figure, 
which he praises. In the African form there are irregular 
costee or lines connecting the radiating lines, in addition to the 
granules, and the granules are small; in the Japan form the 
granules are large, and placed in transverse rows, but there 
are no transverse costa. A slight comparison of Smith’s 
figure with Shadbolt’s or Pritchard’s will make this difference 
obvious. 
In the ‘ Mikro-geologie,’ Ehrenberg figures two species, 
both from earth, from the Island of Camorta, one of the 
Nicobar group. His 4. Jndicus is quite the same as the 
Japan one, having no transverse cost, while the A. Nico- 
baricus seems the same as the African form. 
If there be two distinct species, as is probable, the one 
may be called A. Ehrenbergii, to comprehend the Japan 
species, and that obtained by Dr. Bailey from California 
prior to 1853, as also A. Indicus of Ehrenberg: the other, A. 
formosus, to contain Hemiptychus formosus, Ehr., A. Japoni- 
cus, Shadb., and A. Nicobaricus, Ehr. I have already pointed 
out how these are easily distinguished. For A. Ehrenbergit 
I can only indicate the Japan seas, California, and the 
Nicobar islands, as the localities whence obtained. For A. 
formosus may be assigned a much wider range, as South 
Africa, Nicobar Islands, and West Coast of America. Which 
the British one is I cannot say; I fear there is a mistake 
about its occurring In our seas. 
It is not improbable that of 4. formosus there are several 
varieties ; in some, I find the transverse cost quite simple, 
in others, much and irregularly branched, like the vems of 
the leaf of a dicotyledonous plant, and in a form which I 
have from Mauritius (growing upon Plocamium Telfairia) , the 
radiating coste frequently (if not always with a good light) 
pass between the double row of puncta around the pseudo- 
nodule, and reach the pseudo-nodule itself; this structure 
requires to be verified from other localities ; it seems intended 
by Ehrenberg in his figure of A. Nicobaricus. 
In all that I have examined, taken from off the Alga, the 
lower valve is thinner, and sometimes differently marked 
from the upper one; the characters I have indicated are 
taken from the upper valve only. 
There can be no doubt that these dises have (as said by 
Shadbolt) a horny vegetable outer covering in addition to 
. 
