MEMORANDA. 185 
the two terminal and central nodules of each valve being the 
median line; this line and the central nodule are thus not 
marginal, as hitherto described, but exactly as in other 
diatoms in which such are found. An Amphora would thus 
chiefly differ, by the half of the valve on the one side of the 
median line being concave, while the other was convex; 
whereas, in most genera of the group the two halves of the 
valve are precisely alike. 
The form and structure of the frustule being established, 
the parts capable of affording good distinctive marks for species 
next require to be examined. All naturalists agree, that if these 
are taken from variable parts, they must be of less importance 
than if derived from those that are subject to little or no 
variation; and that no observation can be relied on, of a 
permanent kind, when taken from parts known to change 
their appearance rapidly. Thus, the zone connecting the two 
valves of a diatom, which, from being a mere line, is under- 
stood to attain the whole breadth of the frustule in the course 
of twenty-four hours, has been deservedly rejected; and 
hence it is to be feared that few or none of Dr. Gregory’s 
species of “Complex Amphore,’ which owe their peculiar 
appearance to it, will stand the test of diagnostic characters. 
As the striz, costz, or furrows, are the same on both sides of - 
the median line, and as the valve is folded, those at the back 
of the frustule must be seen through the medium of the sur- 
face nearer the eye, and crossing those belonging to it, so that 
observations on these relate entirely to the accidental position 
the frustule happens to be in. This compels one to depend 
chiefly for essential characters—Ist, on the small portion that 
is seen between the median line and the apparent outline of 
the frustule; and 2d, on the form of the frustule itself, pre- 
vious to the siliceous connecting zone commencing the process 
of self-division. 
No certain conclusions can be drawn as to what is a new 
form or species from deposits or dredgings, on account of the 
impossibility of procuring the species in an isolated state, and 
consequently of studying them independently; the same 
species putting on very different aspects, and different species 
assuming the same aspect at particular stages of self-division. 
Some species of diatoms have both an habitual and an 
accidental appearance. Thus the whole genus Pleurosigma is 
- habitually sigmoid; but P. estuarii, P. strigosum, and some 
others occasionally appear reversed or twisted, both extremi- 
ties being on the same side of the median line; P. reversum, 
fig. 105 of Dr. Gregory’s paper, will illustrate an accidental 
state; the whole supposed new genus Toxonidea of Donkin 
VOL, VI. If 
