SECT. II. 



DIATOMACEJZ. 



199 



or more equal parts, the alternate segments being dif- 

 ferently and highly ornamented, as in the Actinocyclus 

 undulatus (fig. 14), where A is the side view, and B is 

 the front view. The 

 Arachnoidiscus Ehren- 

 bergii takes its name 

 from the likeness of the 

 figures on its circular 

 valves to a spider's web. 

 According to the obser- 

 vations of Mr. Shadbolt, 

 each valve is formed of 

 two superposed layers; 

 on the uppermost of 

 these, which is a thin 

 horny transparent sub- 

 stance, the spider's web 

 is engraven ; and the un- 

 dermost silicious layer, 

 which forms the support- 

 ing frame-work, is like a 

 circular Gothic window. 

 The genus Triceratium, 

 nearly allied to the pre- 

 ceding in general cha- 

 racters, though differing 

 in having a triangu- 

 lar shape, has many 

 species in a fossil state, 

 while others are still 

 existing in the ocean, and in tidal rivers. The Tri- 

 ceratium favus, one of the largest and most beau- 

 tifully marked, occurs in the mud of the Thames, and 

 that of the estuaries of other rivers on our coasts ; it 

 is also frequently found on the surface of uncleaned 



:,, V 

 * * 

 '***< 



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Fig. 13. Pleurosigroa angu- 

 latum. A, entire frustule ; 

 B, its hexagonal areolation; 

 c, the same more highly 

 magnified. 



