114 BRITISH DESMIDIE^. 



Frond deeply constricted at the middle ; segments broader than long ; spines 

 marginal. In a. all the spines are geminate and equal, but in /3. they are 

 more or less unequal and not so regularly geminate, and occasionally also a few 

 are scattered. Mr. Jenner finds, both in the Sussex and the Cornish specimens 

 of this variety, a single spine interposed between the two central pairs and the 

 lateral ones. This I have also observed, but not invariably. The central pro- 

 jections resemble those of Xanthidium aculeatum, but the pearly granules are 

 smaller. I believe that every species of Xanthidium except X. 1 octocorne is 

 punctated, but the puncta can be detected only by the highest powers of the 

 microscope. 



By Brebisson this species is considered the X. aculeatum, Ehr., but Ehren- 

 berg's description and figure represent the spines as scattered. According to 

 Professor Bailey's figure in his * American Bacillaria,' this plant is probably the 

 Xanthidium bisenaritcm, Ehr., but that name is mapplicable to our forms. 

 The number of spines seems to be variable. Bailey's and Ehrenberg's figures 

 have six pairs of spines, Brebisson' s eight, whilst in the Sussex specimens there 

 are generally ten, to each segment. 



Length of frond, not including spines, ^-fr ^^ ^^ ^^^^^ ' mcluding spines, -^^ ; 

 breadth, not includmg spines, ^^^ ; including spines, -g^ ; breadth at con- 

 striction -j-aVo- 



Length of var. /3, not including spines, j^ ; including spines, -^^ ; breadth, 

 not including spines, -^ ; including spines, ^-^ ; breadth at constriction yaVs • 



Tab. XIX. fig. 2. a. front view of typal form ; b. dividing frond of var. (3 ; 

 c. empty frond ; d. end view. 



4. X.fasciculatum (Ehr.) ; segments with 4-6 pairs of subulate, mar- 

 ginal spines ; central projections minute, conical, not beaded, 

 a. Each segment with four pairs of spines. 



Xanthidium fasciculatum, Ehr. Infus. p. 146. t. 10. f. 24. a. (1838). Ralfs, 

 in Annals of Nat. Hist, v. 14. p. 467. t. 12. f. 3. a, h, c, d ; Trans, of 

 Bat. Soc. of Edinburgh, v. 2. p. 154. t. 17. Jenner, Fl. of Tunbridge 

 Wells, p. 194. 



Cosmarium antilopceum, Brebisson, in Menegh. Synop. Desmid. in Linncea 

 1840, p. 218. 



JEuastrum, No. 10, Bailey, Amer. Bacil. in Amer. Journ. of Science and Arts, 

 V. 41. p. 296. t. 1. f. 10 (1841). 



Euastrum fasciculatum, Kiitzing, Phy. Germ. p. 137 (1845). 



Xanthidium polygonum, Hassall, Brit. Freshwater Alg. p. 360, excluding 

 synonyms (1845). 



Xanthidium fasciculatum,var. antilopceum, Brebisson, in lit. cum icone (1846). 



/3. polygonum ; six pairs of spines to each segment. 



Xanthidium fasciculatum /3. polygonum, Ehr. Infus. t. 10. f. 24. b. (1838). 



Ralfs, in Annals of Nat. Hist. v. 14. t. 12. f. 3. e; Trans, of Bot. Soc. 



of Edinburgh, v. 2. p. 154. 

 Xanthidium fasciculatum, Hassall, Brit. Alg. p. 359 (1845). 



a. Common. Dolgelley and Penzance, J. R. Sussex ; and near Southamp- 

 ton, Mr. Jenner. Yate near Bristol, 3Ir. Broome. Kerry, Mr. Andrews, 



