230 BOTANY. 



fineness of some of these markings is astonishing, as will 

 be seen from the following list : 



*Pleurodgma Baltieum 0006 uim. (.000036 incli). 



Pleurodgma angulatain 0005 " (.000019 " 



JVavicula r?wmboides 0004 " (.000015 " 



Amphipleura pellucida 0003 " (.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, eacli containing eeveral families, as follows : 



Tkibb 1. Raphidib^. 



Frustules mostly bacillar (i. e. , lonjrer than broad) ; alwaj s with a dis- 

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

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



Family 1. Cymbellese. Eaplie mostly curved ; valves alike, more 

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



Illustrative genera. Amphora, GymbeUi. 



Family 2. Naviculese. Valves symmetrically divided by the 

 raphe ; frustules not cuiieate or cymbiform. 



Naviculij, (Figs. 154 and 155), Stauronei", Pkurosigma., Amphi- 

 pleura. 



Family 3. Gom.plioiieme8e. Valves cuneate ; central nodule un- 

 equally distant from the ends. 



Oomphonema, Rhoicosphenia. 



Family 4. Achnantheee. Frustules genuflextd ; nodule or gtau- 

 ros on one valve ; mostly stipitate. 



Aehnanthes, Achnanthidium. 



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

 unlike ; valves broadly oval. 



Goasoneis, Anorthein. 



TfiiBB II. Psbudo-Raphidib^. 

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



* Tliese measurements are tlinse given in Carpenter's work on " The 

 Microscope," fifth edition, p. 313. TJiose given by Professor Morley, in 

 Am. Natural st, 1875, p. 439, are a trifle less in each case. 



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

 H. L. Smith, published in I'h'. Len^, 1873-3, and republislied in Le 

 Microscope, si eonstructvm, etc., by Henri Van Heurck, 1878. 



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

 nished by Professor Smith. 



