DIATOMACE^ — ARACIINOIDISOUS. 



291 



is here displayed; and it hehce appears that this marking is 

 superficial, and that the circular areolation exists in a deeper 

 layer of the siliceous lorica. In the alternating divisions whose 

 surface is here displayed, the subjacent areolatjion, when brought 

 into view by focussing down to it, is seen to be formed of equi- 

 lateral triangles ; it is not, however, nearly so well marked as the 

 circular areolation of the first-mentioned divisions. The dark 

 spots seen at the ends of the rays, like the dark centre, appear 

 to be solid tubercles of silex, not traversed by markings, as in 

 many other Diatoms ; most assuredly they are not orifices, as 

 supposed by Prof. Ehrenberg. Of this type, again, specimens 

 are found presenting 6, 8, or 10 radial divisions, but in other 

 respects exactly similar; on the other hand, two specimens 

 agreeing in their number of divisions, may exhibit minute dif- 

 ferences of other kinds ; in fact it is rare to find two that are 

 precisely alike. It seems probable, then, that we must allow a 

 considerable latitude of variation in these forms, before attempt- 

 ing to separate any of them as distinct species. Another very 



Fio. 86. 



Aradmoidiscus Ehrenbergii. 



beautiful discoidal Diatom, which occurs in guano, and is also 

 found_ attached to sea-weeds irom different parts of the world 

 (especially to a species employed by the Japanese in making 

 soup), is the Arachnoidiscus (Fig. 86), so named from the resem- 

 blance which the beautiful markings on its disk cause it to bear 

 to a spider^s web. According to Mr. Shadbolt,' who has care- 

 fully examined its structure, each valve consists of two layers; 

 the outer one, a thin flexible horny membrane, indestructible by 



' "Transactions of Microscopical Society," 1st Series, vol. iii, p. 49. 



