532 PROFESSOR GREGORY ON 
Professor Suir describes the 8.V. as arcuate, as in fig. 107 6; but I find it very 
often quite straight, as in fig. 107. The arcuation seems to be accidental, due only 
to the great slenderness of the frustule, and therefore common; but it is most 
probably naturally straight in the 8.V. as well as on the F.V. Professor BaiLEY 
represents it as straight, although he figures a specimen of the enormous length 
of 0:0265." Those which are not straight are bent quite unequally, some very 
little, others considerably, others only at one end, and others more at one end 
than the other. I feel pretty sure, therefore, especially as straight examples are 
frequent, that it is not essentially an arcuate form. 
This very remarkable species, the longest known Diatom, except a Cheetoceros, 
figured along with it by BaiLey, which is as long, was first observed in this 
country, by me, in the Glenshira Sand, in which, however, I could not find, among 
some hundred specimens, one entire frustule. I figured three fragments, two of 
them nearly complete, in my first paper on the Sand (JJic. Jour., vol. iii., pl. iv., 
fig. 23), and was able to calculate, that if entire, its length would be about one- 
fiftieth or one fortieth of an inch, or 0:02" to 0°025". The length of the specimen 
here figured lies between these measurements, that of Professor BAILEy’s figure is 
alittle above the highest of them. After my paper with the incomplete figures was 
published, I became acquainted with the earlier observations of Professor Barry, 
who had found it living on Sargassum on the American coast. I found one specimen 
of it also recent, but still fractured, before my paper was printed, in a gathering 
made by Professor SmiruH on the south coast. Subsequently, Professor SmirH 
found it frequent in Cork harbour, though smaller than in America. Last year 
(1856) I found it frequent in Professor ALLMAN’s Lamlash Bay dredging, and spa- 
ringly in the other dredgings. As no entire figure of it has yet appeared in this 
country, I have here given two figures, one arcuate, the other straight. 
108. Synedra Hennedyana, n. sp (2) Pl. XIV., fig. 108. This form is in all re- 
spects similar to the preceding, except that the margin is not undulated. Fig. 
108 represents it of the same length as S. wndulata. 
I first noticed this form along with S. wndulata, in July 1856, in Professor 
AuuMaAn’s Lamlash Bay dredging, but I considered it as simply a variety of that 
species. I was led to do so by observing that in S. wndulata it often happens 
that a considerable portion of the margin is devoid of undulations. But several 
other observers who had seen it, adopted the opinion that it was distinct from 
S. undulata. Mr Rover was, I believe, one of these; and I rather think Professor 
Waxxker-Arnort, and Mr Hennepy have come to the same conclusion. Professor 
Arnor? informs me that it occurs in a gathering from the Clyde, | believe near 
Cumbrae, without a single frustule of S. wndulata. As this gathering was made 
by Mr Hennepy, if 1 am not mistaken, and as he has at all events studied the 
form in question, I have figured it under his name, with a mark of doubt, as I 
am not yet quite satisfied that it is really a distinct species. In my material it _ 
