304 T. F. SMITH ON THE VALVE OF TLEDROSIGMA. 



blue " beads," appearing at the edge of the fractures, which 

 prevents a " postage stamp " edge from being seen. I have 

 been fortunate enough to find one valve, and one only, with a 

 fractured end, which clearly shows the red markings to be 

 perforations, and have got it under the microscope to-night for 

 your inspection. Most of the blue " beads " are wanting, but 

 enough remain to show their relative positions and focal planes. 

 And I beg to call your attention to this peculiarity — that the 

 valve is broken through at the end, part in a straight line, and 

 a part at each side sloping outwards. On the straight part are 

 five or six notches inwards, leaving no doubt as to the structure; 

 but where the fracture slopes diagonally are two rows of beads 

 — one on each side — with the bulges outwards. The explana- 

 tion is that the valve is hollow in section, and, the objective 

 being focused on the middle, the structure is penetrated at the 

 sides, and comes upon the inner layer of blue beads. I offer 

 this theory of hemispheres for the middle plane as a possible 

 one only, and one which may explain appearances difficult to be 

 accounted for else. On a slide of these diatoms belonging to me, 

 mounted in balsam, is a crushed valve, showing isolated "beads." 

 To accept them as isolated perforations would be difficult to one 

 not possessing the lively imagination of the Irishman, who 

 explained how they made a cannon by taking a large hole and 

 pouring metal around it. 



One wishes, in treating this subject, they could imitate the 

 confidence of the microscopists of only nineteen years ago in 

 explaining the structure of diatoms. The Rev. J. B. Reade, in 

 his paper on " The True Form of Diatom Markings," when 

 describing the "beads " of Pleurosigma formosum, as shown by 

 his diatom resolver, says: — "Here they are; I can number 

 them, I can weigh them, I can measure them." It is a some- 

 what humiliating fact that in this present year of grace, after 

 the use of a glass such as the then President of the Royal 

 Microscopical Society never dreamt of, I can only offer the 

 presence of beads or hemispheres in one of the focal planes as a 

 possible theory. This, of course, shows one side of the valve 

 only, but I had the honour of showing the delicate membrane 

 on the other side at the last gossip night but one, and to-night 

 produce a negative and some prints for your inspection. The 

 delicate grating, with the round rings, is seen in the prints as a 



