10 Gregory, on a remarkable Group 
On a Remarkable Group of Diatomaceous Forms, witti 
Remarks on Shape or Outline as a Specific Character 
in the Diatomace^. Bj William Gregory, M.D., 
F.R.S.E., Professor of Chemistry. 
(Read October 28tli, 1854.) 
About a year ago I first noticed, in a gathering from Dud- 
dingston Loch, a Navicula of nearly an oval form, with broad, 
obtuse apices, which diiBTered from all the Naviculae known to 
me. The striation was peculiar and strongly marked, the 
striae being about 16 in "001", highly inclined everywhere, 
except just about the middle, where the inclined striae seemed 
to decussate, leaving, of course, a triangular space on each 
side of the centre, while in these triangular spaces the striae 
were parallel and transverse. This arrangement, as we shall 
see, occurs in several species of Navicula and Pinnularia. 
The form here alluded to was referred to different species, 
and even to different genera, by different friends whom I con- 
sulted ; some regarding it as a form of Navicula semen, while 
others supposed it to be related to Pinnularia gracilis, or to 
P. radiosa. But its aspect was totally distinct from that of 
the species named, which, moreover, all have from 24 to 26 
striae in '001'^ 
Meantime other forms occurred, with the same number 
and arrangement of the striae, and with the same peculiar 
aspect, but of different outline. Some were nearly rhombic, 
short, and rather broad ; some were longer, also nearly rhom- 
bic, but with a contraction and subsequent expansion at the 
apices, thus becoming more or less subcapitate. Others were 
nearly linear, with obtuse ends ; others linear and subcapitate. 
Some were found with nearly straight sides, and acuminate, 
ending in small apiculi ; while others had curved sides, con- 
tracted to narrow and produced ends. In some cases these 
produced ends terminated in round knobs ; in others in acute 
points. Some again had straight sides, with contracted ends 
terminating in round heads. 
In the whole of the forms now mentioned, I observed the 
same characters, the same number and arrangement of the 
striae, and the same aspect. On examination it appeared that 
the striae were really moniliform, though not obviously so to 
the eye, having rather a smooth soft aspect than any appear- 
ance of granulation. 
During the whole year new forms, agreeing with those 
already mentioned in every point but that of outline, were 
from time to time observed. In another Duddingston Loch 
gathering, I found in abundance a very fine one, accompanied 
by others, such as those represented in figs. 1, 18, 28, PI. II. 
