110 
Greville^ on Asterolampra. 
Roper^ viz., tliat they are raised and thickened lineS;, intended 
to strengthen the valve. In a frustule of A. Marylandica, to 
which he drew my attention as throwing some light on the 
subject, a portion of the hyaKne area has been accidentally 
destroyed, leaving one of the lines denuded throughout its 
whole length. From its evident strength, it is admirably 
adapted to the purpose assigned to it by Mr. Roper. Indeed, 
the hyaline area, in all these diatoms, may not be unaptly 
compared to a circular window, with radiating bars, fitted 
with transparent siliceous panes. In several species of this 
and the following sections, there is an apparent thickening 
in the rays, especially where they pass the confines of the 
hyaline area. At this part there is sometimes a sort of ridge 
or convexity, which gradually subsides into the plane surface 
of the basal portion of the ray. This appearance is indicated 
in Mr. Shadbolt's figure already quoted, and is probably 
caused by an internal channel, the termination of which, near 
the umbilicus, is occasionally tolerably evident ; but I do not 
find anything like the line passing down the middle of the 
ray, as faintly seen in Mr. Shadbolt^s figure, and conspicu- 
ously in that given by Mr. Brightwell. {' Mic. Journ.,' 
vol. viii, pi. 5, fig. 3.) That the rays are tubular, seems to be 
confirmed by Dr. Wallich, who finds air-bubbles in them in 
some of his balsam-mounted slides. It is now well known, 
that the two valves of the frustules of this group do not cor- 
respond in the disposition of the rays, or, to use a printer^s 
phrase, they do not register ; but that the ray of one valve 
is opposite to the areolated segment of the other, — a circum- 
stance which, to an inexperienced observer, is often not a 
little perplexing. This arrangement was first noticed by 
Miiller, who, in fact, figured the two valves of Asterolampra 
in their normal position, before Ehrenberg himself constituted 
the genus. He remarks, Dieser Stern ist eine Doppelfigur 
und besteht aus einem vordern und hintern stern, woven 
jeder 7 Strahlenhat, die Strahlen des hinteren stehen in den 
Zwischenraumen der Strahlen des vorderen.^^"^ 
It is a curious fact, that in species where the umbilical 
lines are more or less branched, those of the two valves do 
not necessarily correspond. In a beautiful frustule in my 
possession, of Asteromphalus Brookei of Bailey, the upper 
valve is so convex that the umbilical lines can be focused 
distinctly from those of the lower flat valve, and it appears 
that the furcation of the two sets of lines is to some extent 
different. This tends to confirm Mr. Roper^s idea above- 
* Abbandl. d. Berl. Akad. (1841), vol. i, p. 232. 
