79 



On the Structure of Aulacodiscus Margaritaceus, Ralfs. 



By Henry Morland. 

 (Read March 25th, 1SS7.J 



In a paper on "Diatom Structure," which I read before this 

 Club in May last, I gave the following description of the 

 structure of the valves of Aulacodiscus margaritaceus of Ralfs : 

 — "The inner surface consists of a layer with small perfora- 

 tions, some T 2-£otf i ncn across > ^th thickened borders ; above 

 this is a framework of tubes (one to each perforation below), 

 with more or less straight-sided borders. Some of these tubes 

 have 4, 5, or 6 sides, and they are by no means regular in 

 shape. As the outward surface is approached the angles get 

 filled up, and the irregularly-shaped cells end in circular open- 

 ings rather smaller than the cells themselves." 



I have lately been examining this diatom again, but this 

 time under an oil immersion lens, and find that I must modify 

 the above description, which, however, is correct so far as the 

 coarser structure is concerned, but misses the finer details. 



I described the irregularly-shaped cells as ending in " circular 

 openings " on the outer surface. This, I can now emphatically 

 say, was a mistake, these cells really being closed at the outer 

 surface of the valve by a very fine siliceous film with markings 

 very similar to those of Coscinodiscus asteromphalus, Ehr., as 

 figured by Messrs. Nelson and Karop, a year ago, in our 

 Journal. With respect to the small perforations on the inner 

 layer, I find in the examples I have examined that they are 

 furnished with very short tubes projecting inwards towards 

 the body of the frustule, these tubes giving a "ringed" appear- 

 ance to the perforations when the valve is examined direct on 

 the inner surface. As yet I have been unable to detect any 

 film across these inner perforations, but there are Diatomists 

 who seem to be of opinion that these " eye-spots " also are 

 closed by a membrane, though, perhaps, but very slightly 

 silicified. 



Jouen. Q. M. C, Series II., No. 18. 7 



