Objects for the Microscope. 49 



HELIOPELTA, 



From helios (the sun), andpeZfa (a shield). 



This diatom is found in Bermuda infusorial earth. We 

 sometimes find two or three species on the same slide, and 

 few mounted diatoms are as beautiful as the Heliopelta 

 viewed as an opaque with the binocular. We then see the 

 really raised compartments in relief, forming a five or six- 

 rayed star of exquisite workmanship, with a striated margin 

 and lateral spines, which are thought to connect the frustules 

 together when in a young state. The number of rays de- 

 termine the species. If the heliopelta has a five-rayed star 

 in the centre and ten compartments, it is H. Leuwenlwekii. . 

 If there is a six-rayed star and twelve compartments, it is 

 H. euleri. If there is a perfect Maltese cross and eight 

 compartments, this is H. metii. But is quite necessary, 

 in order fully to observe the structure, to have two slides of 

 this diatom, the one for transmitted the other for reflected 

 light, and the latter must have the Heliopelta mounted 

 without balsam. 



OMPHALOPELTA (CELLULOSA VEKSICOLOR). 



This diatom so nearly resembles Heliopelta, that a little 

 close observation is necessary in order to detect the differ- 

 ence between them. The rays, though distinct, are less 

 raised, and the margin has fewer spines, the rim is broader, 

 and one species, 0. versicolor, has, with transmitted light, a 

 play of colour from tawny to red, also a bright, clear, six- 

 rayed star in the centre, and the rim, though narrower than 

 0. cellulosa, is very radiant. This is also found in Bermuda 

 fossil earth, and other species in guano. 



ACTINOCTCLUS, 

 From actin (a ray of light), and cyclus (a circle), 



has no marginal spines, and from eight to ten divisions ; is 

 found alive at Cuxhaven ; fossil in Virginian earth. 



