The Development, de., of Diatomacex. By Dr. G.C. Wallich. 75 
though secreted within the connecting zone of the parent valve, 
may, and actually does, occasionally, excel the latter very consider- 
ably in its dimensions.* When it is mentioned (though of this 
fact I did not apprehend the full significance until I had reaped the 
benefit of Dr. Macdonald’s most valuable observations, eight years 
after I wrote on the subject) that the diameters of the outermost 
“hoop” not unfrequently exceed those of its own parent valve, by 
as much as one-tenth or one-twelfth of those diameters ; whereas 
the thickness of the siliceous wall of the connecting zone is so 
extremely small as well-nigh to baffle the attempt to measure it at 
all; it will, I submit, be conceded that there is strong prima facie 
evidence against the invariable operation of such a law as Dr. Mac- 
donald tries to uphold. 
But this is not all. A cursory examination of a series of 
specimens of the two species of diatoms above named will suffice 
to explain how (I do not pretend to say why) this seemingly 
exceptional character is so marked in them. In both, immediately 
behind and encircling the free margin of each valve externally, 
there runs a deep channel or groove, which consequently projects, 
on the inner side of the valve, into the general cavity, and mate- 
rially curtails its sectional outline on that plane. The connecting 
zone springs, as it were, from the bottom of this channel, then rises 
perpendicularly to it, until well clear of its border ; and then bends 
abruptly forward at a right angle to form the true hoop in each 
connecting zone. Now so little can the connecting zone in 
Biddulphia be said to be “not merely connected but directly 
continuous with the body of its own valve,” forming (as Dr. Mac- 
donald has it) an exact marginal extension of it; and so prone are 
several species of Biddulphia to ring the changes on this part of 
their structure, that in b. turgida the margin of the valve, which 
is generally perfectly even and defined, actually embraces and 
overlaps eaternally the margin of its own connecting zone. In 
this species the overlapped part of the connecting zone consists of 
a gracefully bevelled and broad brim, which curtails 74s own sectional 
outline by as much as one fifth or one-fourth, and abuts against the 
inner side of the concave saucer-shaped valve ; the same result being 
brought about in B. pulchella, by the deep constriction already 
described, in the margin of the valve itself. I would, however, 
specially direct attention to B. turgida, on another ground than the 
presence of the constriction, which in it is very shallow; namely, 
in relation to its at once disposing of Dr. Macdonald’s idea that the 
* It is a noteworthy proof of how often the keen eye of the artist is keener 
than-the keen mind of the scientific investigator whose researches he is illus- 
trating, that in many of the figures in Smith’s ‘ Synopsis’ and in Dr. Greyille’s 
papers on Diatomacee, this and other existent niceties of structure are faithfully 
portrayed. 
VOL. XVII. . G 
