E. M. NELSON ON V LEU RO SIGMA AStiULATUM. 99 



strate the existence of two membranes. At one part the upper 

 membrane had been torn away leaving the lower membrane, 

 at another the lower membrane had gone while the upper was 

 left, the rest of the valve having both membranes in position. 

 These three photomicrographs of the upper, lower and both 

 membranes were exhibited to the Club. No other specimen I 

 have seen has been so fortunately fractured as to demonstrate 

 both membranes so clearly as this one. 



The network in one membrane differs slightly from that of 

 the other, so that after a little practice one is able to state 

 whether the membrane under observation is an upper or lower 

 membrane. The upper membrane in P. strigosum resembles 

 the diamond panes of a leaded light, while the lower is like 

 wire netting, fig. 3. In P. balticum and allied forms the upper 

 memb^ine has slit-like apertures in longitudinal rows, while 



III 



O-go 



Fig. 3. Fig. 4. 



the lower membrane has circular apertures, fig. 4, where the 

 circular apertures in the lower membrane are seen through 

 the intercostal silex of the upper membrane and in a line with 

 the bars between the slits. 



Now at that time it was thought, and probably it is still 

 the received opinion, that the lower membrane " eye-spotted " 

 the upper membrane ; by which is meant that the apertures 

 in the lower membrane are directly below those in the upper 

 membrane much in the same way as in Coscinodiscus the eye- 

 spot is directly below the perforated cap at the top of the cell. 



Recently, however, study on P. angidatum with a Leitz apo- 

 chromat, y 1 ^ inch of 1*40 N.A., has caused me to change my 

 opinion, for the apertures in the lower membrane can be un- 

 mistakably seen below the intercostals of the upper membrane, 

 and this is true not only of P. angidatum, but also of all allied 

 forms that have been examined. 



No mention has been made of Mr. T. F. Smith's observation. 



