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 in Scenedesmus is 

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

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

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

 desmus. 



The endochrome is in general very pale, and starch granules 

 are much less conspicuous than in the other Desmidiece. 



1. 8. 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 Linnsea 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 Leibleini, Kutz. Synop. Desmid. in Linnsea 1833, p. 607. 

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

 and minor, Kutz. /. 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 turgid, and the bristles at 

 their extremities are directed outwards. 



The colouring matter is pale with minute scattered granules. 



In /3. the cells are 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 cseteris plerumque diversa, fines 

 frondis indicat, ct errores a portiusculis earundcm, frequenter oculis occur- 

 sautibus, vitat." — Mencg. Synop, Desmid. p. 20C. 



