BRITISH DESMIDIE.E. 149 



* Empty frond either striate or granulate, generally reddish. 



I. P. margaritaceum (Ehr.) ; frond cylindrical with rotundato-trun- 

 cate ends ; pearly granules in longitudinal series. 



a. Frond fusiform, constricted at the middle ; granules distinct, 



giving a denticulate appearance to the margin. 

 /3. Frond linear, scarcely constricted at the middle ; granules 

 giving a denticulate appearance to the margin, somewhat 

 scattered near the suture. 

 y. punctatum ; frond linear, neither constricted at the suture nor 

 denticulate at the margin ; granules appearing like puncta. 



Chsferium mm-garitaceiim, Ehrenberg, Infus. p. 95. t. G. f. 13 (1838). Me- 

 neghini, Synop. Desmid. p. 236. Jenner, Fl. of Tunhridge Wells, p. 196. 

 Kiitzing, Phy. Germ. p. 132. Hassall, Brit. Freshicater Algce, p. 3/6. 



Penium margaritaceum, Brebissoii, in lit. (1846). 



a. Aslidowii Forest, Sussex, Mr. Jenner. Penzance, /. R. 



/3. Near Bristol, Mr. Thioaites. Wavbleton, Sussex, Mr. Jenner. 



y. Dolgelley, J. R. AshdoTvii Forest, and Henfield, Sussex ; and Reigate, 

 Surrey, Mr. Jenner. Near Aberdeen, Dr. Dickie and Mr. P. Grant. Am- 

 bleside, Mr. Sidebotham. 



Berlin, Ehrenberg. Falaise, Brl-bisson. 



Frond minute, six to twelve times longer than broad, rough with pearly 

 granules which are arranged in longitudmal rows. The forms which I have 

 here considered varieties differ so much, that it is probable they ^ill prove to 

 be distinct species. It mil be seen that the variety /3. is intermediate between 

 the extreme states, and since neither of our British forms agrees with Ehren- 

 berg' s figure and description, I shall describe each in detail. 



a. Frond stout, rounded at the end, distinctly constricted at the middle ; 

 fillets distinct, interrupted and, as well as the endochrone, divided into four 

 equal portions by three pale transverse bands. Near each extremity there is 

 an obscure globule ; segments somewhat fusiform. The empty frond is nearly 

 colourless, and the pearly granules are distinct and give a denticulate appear- 

 ance to the margin. Besides the middle transverse suture, there are two others 

 corresponding wth the interruptions of the endochrome. I have received this 

 form from M. de Brebisson as his Penium margaritaceum. It differs from 

 Ehrenberg' s figure of Closterium margaritaceum in being broader, more con- 

 stricted at the middle, and having more venti-icose segments. The arrange- 

 ment of the endochrome and the position of the globules also differ. 



The variety /?. agrees in colour with the preceding, but it is shorter, and its 

 sides are parallel. The granules are distinct, but less regular. I have only 

 seen it after the endochrome had collapsed. 



The variety y. is elongated, slender, linear, truncate at the ends, and scarcely 

 constricted at the middle. The endochrome is di^dded into four portions by 

 transverse pale bands ; but there are no fillets or globules with moving par- 

 ticles. The empty frond is reddish, especially at its extremities, the last- 

 formed segment being generally paler than the other. The pearly granules 



L 2 



