T. F. SMITH ON DIATOM STRUCTURE. 127 



of specimens of Coscinodiscus, and conlcl not make it fit in any- 

 how. Hitherto I had found all the structure alike, but here 

 was something that upset all my generalizations. 'Nature is 

 variable, but she is not capricious, and it did not seem possible 

 that two diatoms that, seen through a medium power looked 

 so exactly alike, could be constructed on such different prin- 

 ciples. 



Not only, however, are the two layers of structure reversed, 

 but the finer structure of this diatom is missed altogether, 

 and what Messrs. Nelson and Karop have figured as the fine 

 perforations, to me are little bosses standing out from the outer 

 membrane in the position shown, but whether standing on the 

 hexagons and piercing through, or simply standing on the 

 membrane itself, I am unable to determine. These bosses I 

 take to be simply decorative, and agree with other species of 

 Coscinodiscus, and also with Triceratium favus. The dot shown 

 in the centre I cannot account for, except as ;i ghost of the eye- 

 spot just vanishing as the object-glass was focussed down from 

 the wrong side. 



Plus the bosses, I see no difference between Coscinodiscus cen- 

 tralis and asteromjphalos, except that the former is smaller, and 

 has the finer perforations so small as to tax the utmost power of 

 the microscope. On stand No. 2 you will find this diatom, and 

 the disc is in the same position, I imagine, as the one through 

 which Messrs. Nelson and Karop focussed — that is, with the eye- 

 spots upwards. Now, if you work carefully through the disc 

 you will find, first the eye-spots, next the hexagons, then the 

 outer membrane, with the large areolations, the floor of which 

 is filled up with an outer ring of larger perforations, and the 

 centre with twenty or thirty smaller ones ; and, finally, lowest 

 of all, four or five little green bosses that occupy the position 

 that the corners of the hexagons did when in focus. 



Another disc, with the outer side up, shows the finer per- 

 forations distinctly. Another, with little bits chipped out of 

 the centre, will enable you to study it in section ; and the same 

 disc will also show you the little bosses plainly, but this time 

 nearest the eye. I now come to Fig. 11 on Plate IV, which 

 purports to give the finer structure of Aulacodiscus Kittonii, 

 and here, I am afraid, it is not a question of reversing the 

 structure, but misreading it altogether. I may say that I have 



