300 DIATOMACEiE : HELIOPELTA J AEACHNOIDISCUS. 



Parabolic Illuminator (§ 85), which brings into view certain fea- 

 tures that can scarcely be seen by ordinary transmitted light. Five 

 of the radial divisions are seen to be marked-out into circular 

 areolae ; but in the five which alternate with them, a minute granular 

 structure is observable. This may be shown by careful adjustment 

 of the focus to exist over the whole interior of the valve, even on 

 the divisions in which the circular areolation is here displayed ; and 

 it hence appears that this marking belongs to the internal layer* 

 (§ 216), and that the circular areolation exists in the outer layer of 

 the siliceous lorica. In the alternating divisions whose surface is 

 here displayed, the areolation of the outer layer, when brought into 

 view by focussing down to it, is seen to be formed by equilateral 

 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 tuber- 

 cles 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, 10, or 12 

 radial divisions, but in other respects exactly similar ; on the other 

 hand, two specimens agreeing in their number of divisions may 

 exhibit minute differences 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 

 attempting to separate any of them as distinct species. — Another 

 very beautiful discoidal Diatom, which occurs in Guano, and is also 

 found attached to Sea- weeds from different parts of the world 

 (especially to a species employed by the Japanese in making soup) , 

 is the Arachnoidiscus (Plate x.), so named from the resemblance 

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

 spider's web. According to Mr. Shadboltjf who has carefully ex- 

 amined its structure, each valve consists of two layers ; the outer 

 one, a thin flexible horny membrane, indestructible by boiling nitric 

 acid ; the inner one, siliceous. It is the former which has upon it 

 the peculiar spider's web-like markings : whilst it is the latter that 

 forms the supporting frame -work, which bears a very strong resem- 

 blance to that of a circular Grothic window. The two can occasionally 

 be separated entire, by first boiling the disks for a considerable 

 time in nitric acid, and then carefully washing them in distilled 

 water. Even without such separation, however, the distinctness of 

 the two layers can be made-out by focussing for each separately 

 under a l-4th or l-5th inch objective ; or by looking at a valve as 

 an opaque object (either by the Parabolic Illuminator, or by the 



* It is stated by Mr. Stodder ("Quart. Journ. of Microsc. Science," 

 Vol. iii., N.S., p. 215), that not only has he seen, hi broken specimens, the 

 inner granulated plate pi-ojecting beyond the outer, but that he has 

 found the inner plate altogether separated from the outer. The Author 

 is indebted to this gentleman for pointing out that his figure represents 

 the inner surface of the valve. 



t " Transact, of Microsc. Society," First Series, Vol. iii. p. 49. 



