230 BOTANY. 



fineness of some of these markings is astonishing, as will 

 be seen from the following list : 



*Pleurosigma Baltieum 0006 mm. (.000036 inch). 



Pleuroiigma angvlatum 0005 " (.000019 " 



Navieula rlumboidea 0004 " (.000015 " 



Amphipleurapellueida 0003 " (.000008 " 



(a) The classification of DiatomB 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 : 



Tkibb I. Raphidib^. 



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. Cymbelleas. Raphe mostly curved ; valves alike, more 

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



Illustrative genera, Amphora, CymbeUa. 



Family 2. KTaviculese. Valves symmetrically divided by the 

 raphe ; frustules not cuneate or cymbiform. 



Namcida (Figs. 154 and 155), Stauroneis, Pleurodgma, Amplii- 

 pleura. 



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

 equally distant from the ends. 



Oomplumema, Bhoicosphenia. 



Family 4. Achnanthese. Frustules genuflexed ; nodule or stau- 

 Tos on one valve ; mostly stipitate. 



Aehnanthes, Achnanthidium. 



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

 unlike ; valves broadly oval. 



Ooceoneis, AnortJim. 



Tbibe II. Pseudo-Eaphidie^. 

 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. 213. Those given by Professor Morley, in 

 Am. Naturalist, 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 The Lena, 1873-3, and republished in Le 

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



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

 nished by Profespor Smith. 



