82 BRITISH UIATOMACE-(E. 



Genus 27. PODOSPHENIA, Ehr. 



Frustules attached, sessile, cuneate ; valves convex, obovate, inflected 

 at the larger extremity, striated, traversed by a longitudinal median 

 line. 



This genus is placed by Professor Kiitzing at the head of his second 

 tribe, viz. Diatomece vitiates. The Vitta, u^on the presence of which 

 he has constituted this division, do not seem to me to be special 

 organs, but modifications in the outline of the valve, which in the 

 present genus is slightly inflected at its larger extremity, causing on 

 a front view the appearance of notches at the spot where the valves 

 unite with the connecting membrane, and the foramina exist. The 

 apparent prolongation of this notch to the lower extremity of the frus- 

 tule, is nothing more than the valvular suture m hich is seen in all the 

 Diatomaceae. The appearance of two notches at one extremity in the 

 same ft-ustule arises from the progress of self-division ; for on the 

 first formation of the new half frustules, the inflected portions of each 

 valve are in close proximity, and but one notch and one " vitta" can 

 be detected. An examination of the frustules, given in Plate XXIV. 

 figs. 225 and 229, will show the gradual progress of self-division, 

 and the various appearances consequent thereupon. A side view 

 of a single valve after maceration in acid, as in PI. XXIV. 225 «, 

 shows the true form of the valve, and the outline of the inflected por- 

 tion. We have here the first stage in a modification of the form of 

 the valve, which, in its more complete development in Tabellaria and 

 Grammatophora, will remind us of the binate form of many of the 

 Desmidieje, in which the loculi of the cell are united by a narrow 

 connecting isthmus. 



The only very evident distinction between the present and succeed- 

 ing genus, is the presence in Rhipidophora of an elongated and fre- 

 quently dichotomous stipes ; but a close examination of the frustules 

 shows us that the distinct and even moniliform striae, so conspicuous 

 in Podosphenia, are almost wholly wanting in our native species of 

 Rhipidophora. 



1. Podosphenia Ehrenbergii, Kutz. F. V. truncate at the upper 

 extremity; V. acute at both extremities; striae moniliform, 27 in 

 •001". Length -0025" to -0055". v.v. 



