230 BOTANY. 



fineness of some of these markings is astonishing, as will 

 be seen from the following list : 



*Pleurotdgma Balticum 0006 mm. (.000026 inch). 



Pleurodyma angultitum 0005 " (.000019 " 



Navlcula rhomboids 0004 " (.000015 " 



AmpMpleura 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 : 



TKIBE 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. Haphe mostly curved ; valves alike, more 

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



Illustrative genera, Amphora, CymbeUa. 



Family 2. Naviculeee. Valves symmetrically divided by tin; 

 raphe ; frustules not cuneate or cymbiform. 



Navicula (Figs. 154 and 155), Stauroneis, Pkurosigma, Amphi- 

 pleura. 



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

 equally distant from the ends. 



Gomphonema, Rhoicosphenin . 



Family 4. Achnantheae. Frustules genuflexed ; nodule or attiu- 

 ros on one valve ; mostly stipitate. 



Achnanthes, AcJmanthidium. 



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

 unlike ; valves broadly oval. 



Goceoneis, Anortheis. 



TRIBE II. PSEUDO-RAPE IDIE^;. 

 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 Diatomaceae," by 

 H. L. Smith, published in Th: Lem, 1872-3, and republished in Le 

 Micr wope, s i construction, etc., by Henri Van Heurck, 1878. 



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

 nished by Professor Smith. 



