WALKER-ARNOTT, ON ARACHNOIDISCUS, ETC. 203 
as my own, which, indeed, occurred to me from supposing 
Kutzing’s observations to have been misprinted. If, on the 
other hand, he has not inverted these, he must have looked 
on the frustule as a hollow spheroid, and that planes passing 
longitudinally through the central nodules and the eye, 
would cut off the valves. Smith has obviously understood 
Kutzing in this sense, and although some expressions and 
his figures would seem to indicate that he was not altogether 
satisfied on the subject, he adapted his generic character to 
it, and described each valve with the central nodule mar- 
ginal; in this way the whole portion between the median 
lines must be considered as the connecting zone. But that 
such cannot be its real structure is obvious: Ist, from there 
being a deep groove or hollow in front: 2nd, from the sides of 
this groove, nearly up to the median line, being striated 
precisely as on the other side: 3rd, from the median line, 
forming a ridge; the first of these is best seen by putting the 
entire frustule, before being macerated, into balsam; the two 
last require us to examine it obliquely when not in balsam. 
The anterior margin of the valves, where they are attached 
to the connecting membrane, is thus not close to or on a 
level with the nodule, but considerably farther from the eye, 
closer to and more directly above the posterior margin. 
The form of the frustule of Amphora may thus be com- 
pared to that of a coffee-bean, rounded on the back and 
hollowed out in front, a transverse section being somewhat 
reniform or lunate. If we take two circular pieces of paper, 
gum them together by their edges, and mark this disk on the 
margin, with two dots of ink at the extremities of a diameter 
which is at right angles to another which may be supposed 
the axis or line of self-division ; the dots will represent the 
central nodules, and the edge of the disk the median line; a 
diatom lke that would only differ from the genus Navicula by 
the valves being much folded or compressed, while in Navicula 
they are usually nearly flat or depressed. If we now bend 
up the sides of the disk, so that the central nodules approach 
each other, this will in some degree represent an Amphora, 
the only difference being that in the disk the curves of both 
surfaces are almost parallel, the one concave, the other con- 
vex, while in Amphora the anterior or concave one is usually 
smaller than and of a different kind from the posterior. In 
Amphora, then, the median line, as that ought to be called 
which connects the central with the subterminal nodules, 
although apparently marginal, is similar to what is observed 
in other Diatomacez in which nodules exist; indeed, this 
genus only differs by the portion of each valve on the one 
