292 



THE MICROSCOPE AJSTL) ITS REVELATIONS. 



Actinoptychus undulatus : A, side view; 

 B, front view. 



doubted whether this is founded on a valid distinction. 1 These beautiful 

 disks are for the most part obtainable from guano, and from soundings 

 in tropical and antarctic seas. From these we pass on to the genus Acti- 

 noptychus (Fig. 180), of which also the frustules are discoidal in form, 



but of which each valve, instead of be- 

 ing flat, has an undulating surface, as is 

 seen in front view (B); giving to the 

 side view (A) the appearance of being 

 marked by radiating bands. Owing to 

 this peculiarity of shape, the whole sur- 

 face cannot be brought into focus at 

 once except with a low power; and the 

 difference of aspect which the different 

 radial divisions present in Fig. 180, is 

 simply due to the fact that one set is 

 out of focus whilst the other is in it, 

 since the appearances are reversed by 

 merely altering the focal adjustment. The number of radial divisions, 

 has been considered a character of sufficient importance to serve for the 

 distinction of species; but this is probably subject to variation; since we 

 not unfrequently meet with disks, of which one has (say) 8 and another 

 10 such divisions, but which are precisely alike in every other particular. 

 The valves of this genus also are very abundant in the Infusorial earth of 

 Richmond, Bermuda, and Oran (Fig. 191, b, b, #); and many of the 

 same species have been found recently in guano, and in the seas of vari- 

 ous parts of the world. The frustules in their living state appear to be 

 generally attached to Seaweeds or Zoophytes. 



291. The Bermuda earth also contains the very beautiful form (Plate 

 I., fig. 3), which, though, scarcely separable from Actinoptychus except 

 by its marginal spines, has received from Prof. Ehrenberg the distinctive 

 appellation of Heliopelta (sun-shield). The object is represented as seen 

 on its internal aspect by the Parabolic Illuminator ( 105), which brings 

 into view certain features that can scarcely be seen by ordinary trans- 

 mitted 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 

 beaded structure is observable. This may be shown by careful adjust- 

 ment 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 2 ( 289), 

 and that the circular areolation exists in the outer layer of the silicified 

 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 tubercles of silex not traversed by markings, 



1 See Greviilein "Quart. Journ. of Microsc. Science," Vol. vii (1859), p. 158, 

 and in "Transact, of Microsc. Soc.," Vol. viii., N.S. (1860), p. 102, and Vol. x. 

 (1862), p. 41; also Wallich in the same Transactions, Vol. viii. (1860), p. 44. 



2 It is stated by Mr. Stodder ("Quart. Journ. of Microsc. Science," Vol. iii., 

 N.S., 1863, p. 215), that not only has he seen, in broken specimens, the inner 

 granulated plate projecting 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. 



