GREVILLE, ON DIATOMACES. 85 
is widely separated from it in the character of the striation 
alone, to perceive which, requires not only a good object- 
glass, but delicate manipulation. As im many of its con- 
geners the frustules vary greatly in both length and breadth. 
In Canada balsam they become so transparent as to be easily 
overlooked. A. gregoriana must be accounted rare, for 
among the many dredgings and gatherings which have been 
made in Lamlash Bay and its immediate neighbourhood, one 
alone, so far as I know, contains it ; nor is it abundant even 
in that, as seldom more than three or four examples are found 
to occur in a slide. At the same time, there is every reason 
to conclude that its habits must be similar to those of the 
other species, and that a diligent search along the coast 
might be rewarded by its discovery in a living state—para- 
sitic probably on other Algz. 
9. Podosira levis, Greg. MSS. 
Filaments composed of two (?) frustules, which are pale 
yellowish, transparent, glassy, somewhat compressed at the 
poles, very delicately and obliquely striated, and remotely 
and very finely punctate; cingulum firmly siliceous, distinctly 
striated ; diameter of frustule ‘0018” to 0021". (Figs. 15, 
17.) 
Marine. Lamlash Bay; dredged by Professor Balfour, 
1857. 
This conspicuous, although minute little Diatom, was con- 
sidered by my late friend, Professor Gregory, as a Podosira, 
and a few specimens, I believe, were distributed by him 
under the name now adopted. Like P. Montagnei, the fila- 
ments are probably composed of only two or three frustules ; 
but none of the numerous discs or loose valves which I have 
examined, exhibit any indication of the absence of silex, or 
even of imperfect siliceous structure at the apex of the valves. 
Some doubt may therefore be entertained regarding its 
true generic position. The structure, when carefully exa- 
mined under a good objective, is very beautiful, being 
singularly glassy and brilliant, and most delicately striate 
and punctate; characters best seen in the dry-mounted slide. 
In balsam they are not readily perceived. The puncta are 
equally scattered over the whole frustule, and resemble ex- 
cessively minute prominent glands. The peculiarly brilliant 
manner in which the frustules transmit the light renders 
them conspicuous objects in the field of the microscope. The 
species appears to be scarce, having been observed, like the 
preceding one, in a solitary gathering. From three to half a 
dozen frustules not unfrequently occur in a slide; but very 
