230 BOTANY. 



fineness of some of these markings is astonishing, as will 

 be seen from the following list : 



*Pleurosigma Balticum , 0006 mm. (.000026 inch). 



Pleurokigma angulatam 0005 " (.000019 " 



Navicula rlwmboidea 0004 " (.000015 " 



Amphipleura pellucida 0002 " (.000008 " 



(a) The classification of Diatoms is as yet largely artificial. That 

 proposed by Professor H. L. Smith f is one of the most satisfactory ; it 

 is based upon the structure of the frustule. He divides the order into 

 three tribes, each containing several families, as follows : 



TIUBE I. RAPHIDIE^E. 



Frustules mostly bacillar (i.e., longer than broad) ; always with a dis- 

 tinct raphe or median line on one or both valves, and with central and 

 terminal nodules ; without teeth, spines, awns, or processes. 



Family 1. Cymbelleee. Raphe mostly curved ; valves alike, more 

 or less arcuate, cymbiform (i.e., lunate). 



Illustrative genera, Amphora, Cymbetta. 



Family 2. Naviculese. Valves symmetrically divided by the 

 raphe ; frustules not cuneate or cymbiform. 



Namcula (Figs. 154 and 155), Stauroneis, Pleurosigma, Amphi- 

 pleura. 



Family 3. Gomphonemese. Valves cuneate ; central nodule un- 

 equally distant from the ends. 



Gomphonema, Rhoicosphenia. 



Family 4. Achnantheae. Fruetules genuflexed ; nodule or stau- 

 ros on one valve ; mostly stipitate. 



Achnanihes, AcJinanthidium. 



Family 5. Cocconidese. Frustules (generally parasitic) with valves 

 unlike ; valves broadly oval. 



Cocconeis, Anortheis. 



TRIBE II. PSEUDO-RAPHIDIE.E. 

 Frustules generally bacillar (i.e., longer than broad) ; valves with- 



* These measurements are those given in Carpenter's work on " The 

 Microscope," fifth edition, p. 212. Those given by Professor Morley, in 

 Am. Naturalist, 1875, p. 429, are a trifle less in each case. 



f " Conspectus of the Families and Genera of the Diatomacese," by 

 H. L. Smith, published in The Lens, 1872-3, and republished in Le 

 Microscope, sci construction, etc., by Henri Van Heurck, 1878. 



The brief sketch of this system of classification here given is fur- 

 nished by Professor Smith. 



