Naviculacea.] infusorial animalcules. 397 



berg in his great work, (the number of species being then compara- 

 tively few,) contented himself by making two sub-genera, Navicula 

 and Surirella, the former without, the latter with, transverse striae. 

 To these two he has subseijuently added rinmdaria, Stauroneis, and 

 Stauroptera, making in aU five sub-genera of what is called the genus 

 Navicula, but which might, indeed, be called rather the family 

 Naviculea. 



Moreover, not a few species, enumerated in 1838 with Navicula, 

 have been since transferred to other genera — themselves mostly new. 



In framing the characters of his sub-genera of Navicula, Ehrenberg 

 has had recourse to the circumstance of the presence or absence of a 

 median aperture or umbilicus, and its form, in conjunction with 

 that of the presence or absence of transverse striae. Where, on the 

 other hand, that author has entirely transposed species from Navicula 

 of 1838, to other genera, he has been especially guided by the 

 number and disposition of the apertures, coupled vnth the form of 

 the lorica, and its occurrence or non- occurrence in a concatenated 

 manner 



The following plan represents the sub-genera of Navicula, with 

 their mutual relations and distinctions (exclusive of the sub-genus 

 Pleurosigma of the llev. W. Sffuith, — appended.) 



Without a eentral aperture Surirella. 



/ Smooth, or longitudinally ) ^t ■ i 

 ) striped I Navicula. 



Navicula. ■{ ( Umbilicus j 



I rouuded V Transversely striated Pinnularia. 



With a central I 

 aperture oi •< f Smooth or longitudinally ) „. 

 umbilicus. 1 striped ( Stauroneis. 

 I Umbilicus -^ 

 L crucial | Transversely striated Stauroptera. 



This sub-division of Navicula is not approved of by Kiitzing, so far 

 as it rests on the circumstance of the presence or absence of trans- 

 verse strioe, since, as he affii-ms, these striae are variable, and cannot 

 be used as generic characteristics. 



But this writer employs, unhesitatingly, in defining genera, the 

 characters to be drawn from the presence or absence of a median 

 aperture, from its form, and from the position and number of other 

 apertures, as also from the figiuc of the lorica on a trans /erse section, 

 or viewed end-ways. 



