532 PROFESSOR GREGORY ON 



Professor Smith describes the S. V. as arcuate, as in fig. 107 b ; 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 S.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 Chcetoceros, 

 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 (Mic. Jour., vol. iii., pi. 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 002" to 0*025". The length of the specimen 

 here figured lies between these measurements, that of Professor Bailey's figure is 

 a little above the highest of them. After my paper with the incomplete figures was 

 published, I became acquainted with the earlier observations of Professor Bailey, 

 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 Smith on the south coast. Subsequently, Professor Smith 

 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 (?) PI. 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. undulala. 



I first noticed this form along with S. undulata, in July 1856, in Professor 

 Allman'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. undulata 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 Roper was, I believe, one of these ; and I rather think Professor 

 Walkeb-Arnott, and Mr Hennedy have come to the same conclusion. Professor 

 Arnott informs me that it occurs in a gathering from the Clyde, I believe near 

 Cumbrae, without a single frustule of S. undulata. As this gathering was made 

 by Mr Hennedy, if I 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 



