30 W. M. BALE ON SOME OF THE DISCOID DIATOMS. 



or fossil, and it is not found in Ichaboe guano. It is dis- 

 tinguished from the true A. Ehrenbergii by its concentrically 

 undulated valves, by its strong iridescence, and by its sharply 

 defined zones of colour under low powers. Its granules are 

 also more closely and regularly arranged, forming over the 

 greater part of the valve a very regular areolation. To 

 distinguish it from the true A. Ehrenbergii I propose for it 

 the specific name of A. rex. I have only found it in the 

 deposits of Nottingham, Curfield, Atlantic City, and Lyons 

 Creek. Rattray's localities are necessarily unreliable, so far 

 as they are given on the authority of other observers, of whom 

 some at least (Ralfs, for example) were referring to the true 

 A. Ehrenbergii, and not this form at all. 



Rattray's description of this species, however, requires amend- 

 ment, especially as regards the contour of the valve. He says- 

 that large valves have the centre depressed, and two concentric 

 elevated zones between the centre and the border, while small 

 valves have the centre depressed, and are convex between it and 

 the border. This is correct so far as some of the valves are 

 concerned, but in others the surface elevations and depressions 

 are in the opposite order. Thus in large valves the centre is 

 convex, and there is one elevated zone between it and the border^ 

 Evidently the frustule is concentrically undulated as a whole, 

 the depressions of one valve corresponding to the elevations of 

 the other. So in the case of the small valves with depressed 

 centre, others, evidently their counterparts, have the centre 

 convex. Some of the valves in my slides are 0*20 mm. in diameter, 

 Rattray's maximum being 0*17. 



The largest European species is, according to Rattray, A.Ralfsii r 

 of which I have not seen specimens agreeing entirely with 

 Peragallo's description of the type ; but among the forms of 

 A. Ehrenbergii abundant in slides from Cuxhaven and Ichaboe 

 guano are many which agree with that description in the 

 arrangement of the fasciculi and subulate areas, though not in 

 the brilliant appearance, the very large pseudo-nodule, nor the 

 concentric arrangement of the granules. One has only to read 

 the descriptions of Ralfs, Yan Heurck, Rattray and Peragallo 

 to see that no two of these observers agree as to the respective 

 characters of A. Ralfsii and A. Ehrenbergii, which is nob sur- 

 prising if, as Peragallo states, every intermediate gradation 



