DIATOMACE^S. 



413 



Diatomacece, brittleworts, siliceous Bacillaria, are organ- 

 isms composed of two symmetrical plates or valves, narrow 

 or wand-like, navicular — as a miniature boat or "little 

 ship ; " hence their name, Navicula. A rectangular or 

 prismatic figure is, however, the typical form of this 

 family, and the angles of junction of the valves are as a 

 rule, acute. Deeply notched frustules, such as we see in 

 the Desmids, Micrasterias denticulata, Docidium pristidce, 

 Plate II. Nos. 30 and 31, do not occur, and the produc- 

 tion of spines and tubercles is rare among the Diatoms. 

 Each individual Diatom is enclosed by a soft organic 

 matter (sarcode) ; the internal portion is yellowish or 

 orange-brown in colour. In the discoid forms two por- 

 tions are commonly distinguishable, viz. the disc and 

 margin — or rim, and these present different markings, with 

 an occasional central prominence, called an umbo or boss. 

 Great variety of outline may prevail in a genus, so much 

 so, that no accurate definition can be safely 

 laid down : thus in the genera Navicula, 

 Pinnularia, the frustules are in one aspect 

 boat-shaped, and in another oblong with 

 truncated ends, prismatic. Mr. Brightwell 

 thus describes and explains the transitions 

 of form produced by a change in position 

 of the frustules of the genus Triceratium. 1 

 " The normal view of the frustule may be 

 represented by a vertical section of a tri- 

 angular prism. If the frustule be placed 

 upon one of its flat sides, we look down 

 upon its ridge and obtain a front view of 

 its two other sloping sides. If it be placed 

 upon one of its ridges, we have a front 

 view of one o£ its flat sides, generally 

 broader than long, and of its smooth or p . g 

 transparent suture or connecting mem- Zema ehngatum, 



brane. If the frustule be progressing 



aud capitatum. 



towards self- division, it is then often considerably longer 

 than broad, and when nearly matured for separation, pre- 

 sents the appearance of a double frustule." So with re- 



1) Journ. Micros. Soc. voL I. p. 248. 

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