THE ANNALS 



AND 



MAGAZINE OF NATURAL HISTORY, 



[SECOND SERIES.] 



«« per litora spargite museum. 



Naiades, et circiJra vitreos considite fontes : 

 Pollice virgineo teneros hic carpite flores : 

 Floribus et pictum, divae, replete canistrura. 

 At vos, o Nymphae Craterides, ite sub undas ; 

 Ite, recurvato variata corallia trunco 

 Vellite muscosis e rupibus, et mihi conchas 

 Ferte, I)e» pelagi, et pingui cqnchylia succo." 



N.PartkeniiGiannettasHEcUl. 



No. 49. JANUARY 1852. 



I. — Notes on the Diatomacese ; with descriptions of British spe- 

 cies included in the genus Pleurosigma. By the Rev. Wm. 

 Smith, F.L.S. 



[With two Plates.] 



[Continued from vol. vii. p. 14.] 



In all systematic descriptions of the Diatomacea, the genus Na- 

 vicula occupies a prominent place ; in some arrangements, indeed, 

 the terms Naviculacece or Naviculece are adopted as the designa- 

 tions of the entire tribe, in others as the titles of an extensive 

 family, and, even when most circumscribed, embrace a vast 

 number of closely allied species whose distinction and identifica- 

 tion present grave difiiculties to the progress of the systematist. 

 In Prof. Kiitzing's latest work on the Diatomacece (Species Alga- 

 rum, 1849), the genus iV«z;zcM/« includes 174 species; and when 

 it is remembered that these minute organisms require the careful 

 employment of the microscope for their detection, and vary from 

 each other by diversities in form so slight, or valvular markings 

 so delicate, that the highest powers of our best instruments are 

 called into constant requisition, it will not appear remarkable 

 that the most judicious descriptions, aided by carefully prepared 

 Ann. ^ Mag. N, Hist. Ser. 2. Vol.ix. j\ 



