130 T. F. SMITH ON DIATOM STRUCTURE. 



exhibit the eye-spots nearest the eye, and yon may observe 

 how exactly alike the appearance is to that of Coscinodiscus. 



There now remains, first, the question of the position of the 

 perforated membrane, which is found on one # side of most 

 diatoms having the hexagonal structure ; and, second, whether 

 the eye-spot, which is always found on the other side, is closed 

 with a membrane or remains a large perforation. With regard 

 to the former, my own opinion is that the fine perforated 

 membrane is generally stretched outside the • hexagons ; and 

 sometimes at a considerable distance from it. I have five 

 slides here to-night that seem to me to confirm this view, two 

 of which I have described, and it remains to describe the other 

 three. 



On stand No. 3 is a Coscinodiscus from Richmond, Virginia, 

 having a spine standing out at each corner of the hexagons, 

 and considerably above them, but just within the point of the 

 spines is stretched a very fine membrane. The specimen is 

 torn, and you will see the free edge of the membrane projecting 

 beyond the hexagons. 



On stand No. 4 is a Triceratium, in which the fine membrane 

 is chipped off from over parts of three or four hexagons, and if 

 you study the torn edge of the membrane I think you will see 

 that it is altogether outside. 



On stand No. 5 is a Triceratium with the raised margin of 

 one side chipped away, and here also, I think, a study of the 

 torn edge will lead you to the same conclusion. At one spot the 

 hexagons project beyond the torn edge of the membrane. 



On whether the eye-spot has a closing membrane in all cases 

 I offer no definite opinion; but I have two slides which seem 

 to prove that in some instances there is one. 



In the same slide, from the Oamaru deposit, is a half disc of a 

 Coscinodiscus, and there is no doubt about the closing membrane 

 there ; but what is curious about it is, that the centre of each 

 is granulated as if the process of wearing away was begun. In 

 other parts the centre is wanting, but the eye-spot has no 

 thickened edge. One other slide I have with a specimen of a 

 smashed Coscinodiscus from Jutland. The eye-spots are nearest 

 the eye, ami in one of the spots is a line membrane torn across, 

 showing I lie edge most distinctly. This is too delicate an 

 object to see well with an Abbe condenser. 



