107 



THE SECONDARIES OR DOTTED STRUCTURE IN 



PINNULARIAE. 



By Chapman Jones. 

 {Read April 25th, 1916.) 

 Communicated by A. Morley Jones. 



Plate 7. 



About forty-five years ago Mr. Henry J. Slack contributed a 

 note to the Royal Microscopical Society * in which he remarks 

 that " at one time it was supposed that the Pinnulariae were 

 distinguished from allied forms by solid costae replacing beaded 

 bands," but " more recently this distinction has not been deemed 

 valid." He continues that " an examination of the Pinnulariae 

 on Moller's type-slide led to the belief . . . that instead of 

 broad irresolvable ribs, a truer view " of the costae " exhibited 

 fine lines of beads springing from the median band, with a furrow 

 between them, and in that furrow another line of beads at a 

 lower level." Mr. Slack obtained from Moller a series of Pinnu- 

 lariae mounted dry, and examined them by means of " a re- 

 markably fine immersion l/8th by Powell & Lealand on their 

 new system. The condenser employed is a 4/lOth one of Ross, 

 and the usual stop, one radial slot, aperture 109°. The most 

 serviceable eyepieces were C and D of Ross's scale." The twelve 

 species examined all showed, though some with much difficulty, 

 " the composite character of the so-called costae." 



Quite recently, Mr. T. A. O'Donohoe f has produced a photo- 

 graph of a Pinnularia nobilis which shows excellently the presence 

 of a dotted structure. He used annular illumination and a 

 2 mm. apochromat. But in spite of these results, and perhaps 



* The Monthly Microscopical Journal, xxxii., August 1871, p. 71. 

 I Journal of the Quekett Microscopical Club, April 1914, p. 309. 



JouRN. Q. M. C, Series II.— No. 79. 9 



