34 cocconiYCEJE. 



var. c. dimoxphus. Rabh. Alg. ill.. 63. 



Cells acute, 4-8, placed evenly in a single row ; inner cells 

 fusiform, outer externally lunate. 



Scenedesmus dimorphus, Ralfs. Ann. Nat. Hist xv., p. 403, 

 t. 12, f. 5. Brit. Desm. p. 191, t. 31, f. 13. Hass. Alg. 393, 

 t. 92, f. 13. 



Formerly the typical form and its two varieties were regarded as three 

 separate species, but there scarcely seems sufficient reason for thus re- 

 taining them. Ralfs wrote of them long ago, " When the cells are nearly 

 uniform >$. acutus has some resemblance to 8. dimorphus ; but in the 

 latter the cells are more slender, never ventricose, and are arranged 

 quite evenly side by side. It is more difficult to distinguish S. acutus 

 from 8. obliqiius, and I am far from certain that Ehrenberg erred in 

 uniting them. The principal distinction is that in 8. acutus the cells 

 form only a single series, which is nevertheless irregular, on account of 

 the alternate projection of the cells in opposite directions. In S. obUquus, 

 on the other hand, the cells by division form two distinct rows, which, 

 after separation, become two fronds." These characteristics are better 

 shown in Ralfs' figures than in our own, which are more intermediate, 

 and show an evident approximation to the typical form. 



Plate XIII. fig. 6. a, cells of the typical form ; b, cells of the variety 

 dimorphus; c, of the variety obliquus. All magnified 400 diameters. 



Scenedesmus antennatus. Breb. Rolf. Desm. t. 35, /. 27. 



Cells fusiform, 2-4-8, joined in a single or double series, all 

 somewhat curved, usually ventricose, cuspidate at each extremity, 

 the apices bearing a hyaline globule. 



SIZE. Cells 0025--Q035 mm. diam., -013 mm. long. 



Rabh. Alg. iii. 63. Archer in Pritch. Infus. p. 753. 



In pools. 



" Scenedesmus antennatus resembles S. acutus in form, and also in the 

 arrangements of the cells ; but is distinguished from that and every 

 other species by having the attenuated points tipped by minute glo- 

 bules." Ralfs. 



At the time the above was written it had not been found in this 

 country, and is still the rarest species, if its specific identity can be 

 maintained. 



Plate XIII. fig. 7. Cells magnified 400 diameters. 



B. Cells armed. 

 Scenedesmus quadxicauda. Breb. Ralfs Desm. 190, t. 31, f. 12. 



Cells oblong-cylindrical, each extremity obtusely rounded, 

 2-4-8, narrowly united, either in a single or double series, all 

 straight, the outer cells at each end (and rarely some of the 

 intermediate ones) armed at each extremity with a recurved 

 spine. 



SIZE. Cells -0085-*01 mm. diam. to -022 mm. long. 



Rabh. Alg. iii. 65. 



Scenedesmus quadricaudatus, Ralfs Ann. Nat. Hist, xv., p. 

 402, t. 12, f. 4. Hass. Alg. 392, t. 92, f. 12. Jenner Fl. Tun. 

 Wells, p. 200. 



In standing water. 



