160 T. A. O'DONOHOE ON COSCINOD1SCUS ASTEROMPHALUS, ETC. 



The second slide shows the round black clots of the inner 

 membrane near the nodule, where the outer membrane has been 

 rubbed off (PI. 14, fig. 1). The third slide shows an impression 

 of the greater part of a valve caused by the adhesion of the outer 

 membrane to the slip. 



The fourth slide shows a similar adhesion to the cover-glass, 

 as well as the valve from which the outer membrane was torn. 

 The fifth shows the same x 1,000. 



The sixth slide shows fine hair-like bent fibrils breaking away 

 from the valve. 



The seventh shows a part of the same valve x 2,000, on which 

 four fibrils of the inner membrane are visible. 



The eighth is an image which puzzled me, and Mr. Nelson 

 kindly explained it thus : " This shows an upper bar crossing a 

 hole. It also shows the transverse girder work wonderfully 

 clearly" (PI. 14, fig. 2). The ninth slide shows the structure of 

 the inner membrane to be similar to the last, but this and the 

 three next following photographs were taken from Mr. Nelson's 

 slide. 



The tenth shows the outer membrane breaking up into fibrils, 

 and sometimes even into isolated clots (PI. 14, fig. 3). 



The eleventh and twelfth show continuations of the tenth. 

 The thirteenth shows the structure of the fibrils very well (PI. 14, 

 fig. 4). The fourteenth and fifteenth show the kind of structure 

 which is incorrectly taken for squares, but a glance at one of 

 the single fibrils causes the optical illusion to vanish. 



In Pleurosigma balticum the fibrils run parallel with the raphe, 

 whereas in Pleurosigma angtdatum they seem to run obliquely to 

 the raphe, and this, it seems to me, is the chief difference in the 

 minute structure of the two valves. 



Description of Plate 14. 



Fig. 1. P. balticum, X 1,750. Showing the structure of the 

 inner membrane when the outer has been rubbed off. 



,, 2. P. balticum, x 2,000. Inner membrane, showing the 

 holes crossed by very fine bars of silex. 



3. P. balticum, x 2,000. Fibrils, photographed from Mr. 

 Nelson's slide. 



,, 4. P. balticum, x 1,250. Fibrils. 



Journ. Quekett Microscopical Club, Ser. 2, Vol. XII., No. 73, November 1912. 



