290 MICROSCOPIC FORMS OF VEGETABLE LIFE. 



the appearane of being marked by radiating bands. Owing to this 

 peculiarity of shape, the whole surface cannot be brought into 

 focus at once, except with a lower power; and the difference of 

 aspect which the different radial divisions present in Fig. 84, is sim- 

 ply 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 con- 

 sidered a character of sufficient importance to serve for the dis- 

 tinction 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 so pre- 

 cisely alike in every other particular, that they can scarcely be 

 accounted as specifically different. The valves of this genus are 

 very abundant in the infusorial earths of Richmond, Bermuda, 

 and Oran (Fig. 101, 5, 5, ft) ; and many of the same species have 

 been found recent in guano, and in the seas of various parts of 

 the world. The frustules in their living state appear to be ge- 

 nerally attached to sea- weeds or zoophytes. 



182. The Bermuda earth also contains the very beautiful form 

 (Fig. 85), which, though scarcely separable from Actinocyclus 



FIG. 85. 



Heliopdta. 



except by its marginal spines, has received from Prof. Ehrenberg 

 the distinctive appellation of Heliopelta (sun-shield). In the re- 

 presentation here given, the object is delineated as seen by the 

 parabolic illuminator ( 61), which brings into view certain fea- 

 tures that can scarcely be seen by ordinary transmitted light. 

 Four of the radial divisions are seen to be marked out into 

 circular areolse; but in the four 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 of 

 the valve, even on the divisions in which the circular areolation 



