403 Mr. J. Ralfs on the British Desmidiese. 



some writers, by placing too much reliance on the arrangement 

 of the cells, have raised varieties to the rank of species*. 



Scenedesmus differs from the preceding genera in the very dif- 

 ferent form of its cells, but Pediastrwn forms a connecting link 

 between them. As in that genus, the frond m Scenedesmus is 

 composed of several cells, but diflerently arranged ; and the di- 

 vision into two segments, which, although modified, is still met 

 with in the outer cells of Pediastrum, is enthely absent in Scene- 

 desmus. 



The endochrome is in general very pale, and starch granules 

 are much less conspicuous than in the other Desmidiece. 



1. <S. quadricaudatus, Breb. Cells generally four, oblong, rounded at 

 their ends, disposed in a single row ; each extremity of the two 

 external ones terminated by a bristle. Breb. Alg. Fal. p. 66 ; 

 Menegh. Syn. Desmid. in Linna;a 1840, p. 206. Arthrodesmus 

 quadricaudatus, Ehr. Infus. p. 150. tab. 10. fig. 16"; Pritch. Infus. 

 p. 189. 



/3. External cells with a bristle at each extremity, and one at the 

 centre of the outer margin. 



y. ecornis, Ehr. All the cells similar and without bristles. Scene- 

 desmus Leible'mi, Kutz. Synop. Desmid. in Linnsea 1833, p. 607. 

 fig. 98 ; Menegh. I. c. p. 207. Scenedesmus bijugatus, trijugatus 

 and minor, Kutz. I. c. p. 607. figs. 97 and 99. 

 In pools not uncommon. Storrington Common and Eastbourn, 



Sussex, and Shoreham, Kent, Mr. Jenner ; Cheshunt, Mr. Hassall ; 



Dolgelley. 

 /3. Dolgelley. 

 y. Weston Bogs near Southampton, Mr. Jenner ; Bristol, Mr. 



Thwaites ; Dolgelley and Penzance. 



Fronds composed of from four to eight oblong cells, which are 

 generally larger than in any other species of this genus ; they are 

 about three times as long as broad and rounded at their ends j 

 the external ones are usually more tm-gid, and the bristles at 

 their extremities are directed outwards. 



The colom'ing matter is pale with minute scattered granules. 



In |3. the cells ar£ smaller, and the external ones, besides the 

 usual terminal bristles, have another from the centre of the outer 

 margin. 



The variety y. is described by several authors as a distinct spe- 

 cies, but I agree with Ehrenberg in considering it a state of this 



* " Distributio cellularum constantissima et characteristica, forma in spe- 

 ciminibus bene evolutis pro unaquaque specie semper eadem." 



" Forma cellularum extremarum, a casteris plerumque diversa, fines 

 frondis indicat, et crrores a pordusculis earundem, frequenter oculis occur- 

 santibus, vitat." — Meneg. Synop. Desmid. p. 206. 



