WALKER-ARNOTY, ON ARACHNOIDISCUS, ETC. 201 
stricted frustule and carinate valve or median line; the first 
of these is not to be much relied on, as it is exhibited slightly 
in some species of Navicula, while it is observed only in a 
modified manner in A. vitrea. In this species, and also in 
A, elegans, there seem to be frequently four valves to the 
frustule ; I have seen the same structure in Schizonema cruci- 
gerum, Pinnularia major, lata, alpina, and some others, but 
it is accidental and not characteristic of such species. 
Dr. Gregory, in his paper on ‘ Clyde Diatoms,’ introduces 
into the descriptions of some of his forms, a notice of a “ plate” 
which is said to be above the valves; he does not seem 
to consider its occurrence as universal in the genus, but 
characteristic of certain species only. How these plates 
are formed or attached I do not understand. I am not 
aware that they have been seen separated from the valves, 
and if a peculiarity of structure of the valves, they ought to 
be traced in these. Although, however, I have attempted to 
model in clay or putty a frustule with the valves as figured 
and described, connected by a zone, I have not succeeded in 
constructing anything like these plates. That there is such an 
appearance is unquestionable, so that a suspicion arises that 
such plates do not lie above the valves, and indeed have no 
actual existence, but that the whole arises from our seeing 
the margin, or the surface outline of the valve, through the 
medium of part of the frustule. If proved to be actually 
external and above the valves, or to be caused by any 
other difference of structure from what is usual in the genus, 
its importance as a specific distinction will be readily allowed. 
I exclude, at present, from the genus the curious and perfectly 
distinct A. complexa of Dr. Gregory. As the slices are not per- 
forated, they cannot be annuli, such as occur in Rhabdonema ; 
and if entire lamina of the connecting zone,* an obscure affinity 
with Rhipidophora would be established. The true structure of 
this diatom, and therefore its genus, is however quite uncertain. 
* In various species, particularly marine ones, of different genera of 
diatoms, as Navieula, Amphora, Amphiprora, and Schizonema, the connecting 
zone exhibits frequently a lamellar structure, the number and appearance 
of the lamellze varying much in the same species according to circumstances. 
Navicula Libellus of Dr. Gregory, on that account, seems to be the well-known 
and not uncommon state of Schizonema Grevillii. When these lamelle are 
perfect, the frustule is necessarily divided by them into two cells, a proof 
that it is then undergoing self-division; but the cause of this appearance, 
or why it is not to be detected at all times in the same species, is unknown 
to me. Several, if not all, of Dr. Gregory’s group of *‘ complex Amphoras” 
are in this predicament ; at least in these the lamelle are not described and 
figured as annular or with a perforation, as would be were the zone not in 
the variable or transition stage, and thus unsuited for specific distinctions. 
