343 



Mr. T. F. Smith read a paper on the " Structure of the Valves of Pleuro- 

 sigma" 



Mr. E. M. Nelson said he felt greatly obliged to Mr. Smith for his paper, 

 ■which he considered to be of high value, and having seen the specimen 

 described, he could quite confirm what had been said about it. He was the 

 more glad to hear the observations of Mr. Smith, because he thought that 

 to go hunting again and again over these specimens was the only kind of 

 work of any real value. No donbt Mr. Smith had proved that there were 

 several of those membranes, although it had not been possible to follow all 

 that was contained in the paper the first time of hearing it. The difficulty 

 had always been felt that in Formosv.m they never had been able to see the 

 fracture passing through the holes ; but now it had been shown that there 

 was undoubtedly a perforated membrane ; they could take it and see for 

 themselves that it was a perfectly plain thing ; indeed, if this was not 

 real, then he could only say that all other things were " hocus pocus." 

 With their very best lenses they had tried to find out about Formosum. 

 Mr. Smith had found this fracture, had shown it to him, and demonstrated 

 that the membrane existed, and that at any rate the fracture did run 

 through the holes. There was still great trouble in determining the struc- 

 ture of Ayigtrfatum, which gave some most extraordinary appearances 

 under some conditions. They hoped, however, to be able to understand it 

 at some future time, especially now that Mr. Smith seemed to have cleared 

 up some very difficult points in his paper, which he thought was one likely 

 to be of great value. 



Mr. A. D. Michael said the subject was a good deal out of his particular 

 line, but he recollected a German paper being referred to some time ago in 

 the " Journal of the Eoyal Microscopical Society," in which the author con- 

 tended for a structure very much the same as that described by Mr. Smith, 

 namely, an inner and an outer membrane or layer ; the outer was said to 

 be entirely without markings, and plane on its surface, and that the holes 

 were entirely on the inner plane. This was said to be the reason why the 

 cast of a diatom did not show any raised markings. Two membranes with 

 a grating between and the markings on the inner membrane, was, so far as 

 he recollected the paper, the idea of the structure. 



The President said that the subject was quite out of his depth, for he 

 had not paid much attention to diatoms, but he could only listen with 

 admiration at the patience with which Mr. Smith had followed up his investi- 

 gations. With regard, however, to the perforations which he had described, 

 and which would appear to be about 50 ^ 00 inch in diameter, he thought 

 they were talking about something which might or might not have an 

 existence, but about which there was no certainty whatever. Of course it 

 would not do to say they did not exist, but for his own part he did not 

 believe that a ray of light could be passed through such a structure as that 

 which had been described, and that afterwards it could be gathered up 

 again so as to produce a reliable image of that structure. Taking, for 

 example, the eyes of insects, the corneal lenses were, of course, qnite out- 

 side the region of doubt, because being not much less than the tqoo mcn m 



