166 e. m. nelson on the nodules in pleurosigm^. 



Climacosphenia Moniligera. 



There is another diatomic structure, quite unconnected with 

 the scope of this paper, but which nevertheless I would like to 

 bring to the notice of the Club, as it is one of the most beautiful 

 and interesting, as well as instructive, of microscopic structures. 

 This is a diatom known as a Climacosphenia moniligera. The 

 principal view of the valve shows an elongated isosceles triangle, 

 having three bands running its whole length. The outer bands 

 are sieve- like structures ; the minute holes being closer together 

 in the transverse than in the longitudinal direction. 



In common parlance it would be said that the longitudinal 

 strise were finer than the transverse ; the transverse stride vary, 

 however, being finer at the wide end of the valve, where they 

 count 53,000 per inch, and coarser at the small end. This agrees 

 with the law of diatom formation in circular forms I have on 

 several occasions pointed out — viz., '* immatui-e at the centre, 

 mature at the periphery " ; the growth of circular forms being 

 from the centre outwards. The climacosphenia grows from the 

 base or small end towards the apex or wide end. 



The middle band has coarse structure at the rate of 33,500 

 per inch. It takes a fine 12 mm. apochromatic to dot the outer 

 and finer bands near the apex or large end in a balsam mount, 

 with axial cone illumination. When this diatom is seen sideways 

 it shows the interesting structure alluded to above ; this consists 

 of a helical pipe passing through some bulkheads, which divide the 

 diatom into several compartments. As we come to the com- 

 partments near the wide end, or older part of the valve, we find 

 that the pipe closes up and the bulkheads become solid silex. 



Mr. Rousselet has appropriately suggested that this structure 

 is a useful test for 12 mm. apochromatics, and similar objectives. 

 With such glasses, however, the helical tube appears only like 

 interlockmg teeth, it requires more aperture to develop its full 

 beauty. In conclusion, let mo say that this is an object that will 

 repay careful examination. 



I must express my indebtedness to Mr. Morland, not only for 

 the loan of slides from his unrivalled collection, but also for his 

 kind assistance in the preparation of this paper; and also to 

 Mr. E. T. Newton for geological information. 



