1899] MICROSCOPICAL JOURNAL. 9 



is strictly in accordance with the law. of diatomic growth 

 viz., mature at the periphery, immature at the centre. 



Although these forms have specific names allotted to 

 them, I regard them as being early stages in the evolu- 

 tion of the Asteromphalus. 



With regard to the eye-spot layer, it is altogether ab- 

 sent in the first stages, but makes its appearance when 

 the primaries assume their hexagonal or polygonal shape. 

 The terminus ad quem may be said to be a circular plate 

 of silex sparsely perforated with circular primaries. 



DISCUSSION. 



Mr. Nelson said he was indebted to Mr. Ing-pen for this 

 beautiful diatom slide, the material of which came from the 

 Nottingham (Maryland) deposit. It was just the sort of 

 slide he liked to get hold of for this purpose, with the dia- 

 totns smashed up all over the slide. 



Mr. Ingpen said the slide had been sent to him as a sam- 

 ple of mounting in high refractive media ; he believed in 

 this case the medium was antimony. The deposit itself 

 was a very interesting one. Professor Hamilton Smith 

 at that time was experimenting a great deal on media of 

 high refractive index, such as bromide of antimony, 

 bromide of arsenic and piperine, giving an index of a little 

 over 2, and they certainly made the objects mounted in 

 them very distinct. There was another thing about this 

 particular culture worth mentioning, and that was, it had 

 never been doctored to any great extent, and therefore the 

 structure stood well in the process of mounting; and the 

 fact of Mr. Nelson finding these tertiary structures was 

 no doubt due to the material not having been over-washed. 

 When mounting in these high refractive media he always 

 found this particular culture was much more successfully 

 dealt with than any other. He had not mounted these 

 slides for the sake of the beauty of the specimens, but 

 rather to see the effects of the different media, and it was 

 found almost impossible to mount many of these forms 

 without fracture. Mr. Ingpen thought it highly probable 



