Greville_, on New Diatoms. 
47 
close^ with faint rays formed by pairs of tlie longest lines. 
Diameter -0045''. 
LlRADISCUS. 
Liradiscus mmutuSy n. sp., Grev. — Frustule minute, nearly 
sphserical. (Fig. 6.) 
Hah. Barbadoes deposit, Cambridge estate; in slides com- 
municated by C. Jobnson, Esq. 
This will be recognised at once by its small size, the 
diameter being only from '0007"' to -0010''. When viewed 
in front it is exceedingly like a Cresswellia, except in the 
connecting zone. 
Arachnoidiscus. 
Arachnoidiscus Grevilleanus, n. sp., Hardman MS. — Disc 
with the concentriq circles of the cellules continued uninter- 
ruptedly to the centre, the umbilical point being filled up 
with a rosette of a few very minute linear cellules, (Fig. 7.) 
Hab. Barbadoes deposit, Springfield and Cambridge es- 
tates; Laurence Hardman, Esq. ; C. Johnson, Esq. ; B. K. G.; 
rare. 
The few species of this exquisite genus are in such con- 
fusion that I cannot venture at the present moment to dis- 
entangle them. It is doubtful how far the characters by 
which they are distinguished are really of value. Possibly 
some mistakes may have originated in consequence of differ- 
ences between the upper and under valves having been over- 
looked. The granules (or cellules) which surround the um- 
bilicus are subject to great variation; and the number of 
radiating ribs, both long and short, appear to depend on the 
size of the valve. With regard to the species now under 
consideration, on which my friend Mr. Hardman has done 
me the great honour to bestow my name, it difi'ers so essen- 
tially from every other so-called species that 1 cannot hesi- 
tate to admit it. It does not even correspond with the defi- 
nition of the genus itself, which attributes to the disc a cen- 
tral hyaline nodule or umbilicus.^^ In our new diatom 
there is not the remotest trace of anything of the kind. 
The long radiating ribs penetrate nearly to the centre, where 
the cellules begin to lose their concentricity, and to become 
gradually smaller, the central point itself being occupied by 
a little star composed of about six very minute, linear, 
truncate cellules. It is satisfactory that specimens should 
have been obtained from diff'erent sources: from the de- 
