DIATOMACEvE 



difference of aspect which the different radial divisions present in 

 fig. 456 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 lias 

 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 discs, of which one has (say) 

 eight, and another ten such divisions, but which are precisely alike 

 in every other particular. The valves of this genus also are very 

 abundant in the infusorial earth o Richmond, Bermuda, and Oran 

 (fig. 467, b, b, b), and many of the same species have been found in 

 guano and in the seas of various parts of the world. The frustules 

 in their living state appear to be generally attached to seaweeds or 

 zoophytes. 



The Bermuda earth also contains the very beautiful form 

 which, though scarcely separable from Actinoptychus except by 

 its marginal spines, has received from Professor Ehrenberg the dis- 

 tinctive appellation of Heliopelta (sun shield). The object is repre- 

 sented as seen on its internal aspect by the parabolic illuminator, 

 which brings into view certain features that can scarcely be seen by 

 ordinary transmitted light. 

 Five of the radial divisions are 

 seen to be marked out into 

 circular arealse ; but in the 

 five which alternate with them 

 a minute beaded structure is 

 observable. This may be 

 shown, by careful adjustment 



of the focus, to exist over the 



, , . , '. , FIG. 456. Actinoptychus undulatus . 



whole interior of the valve, A, side view ; B, front view, 



even on the divisions in which 



the circular areolation is here displayed ; and it hence appeal's pro- 

 bable that this marking belongs to the internal layer, 1 and that the 

 circular areolation exists in the outer layer of the silicified valves. 

 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 end of the 

 rays, like the dark centre, appear to be solid areolations of silex not 

 traversed by markings, as in many other diatoms ; they are appa- 

 rently not orifices, as supposed by Professor Ehrenberg. Of this type, 

 again, specimens are found presenting six, eight, ten, or twelve 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 



1 It is stated by Mr. Stodder (Quart . Journ. Microsc. Science, vol. iii. n.s. 1863, 

 p. 21")) 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. 



R R 2 



