116 BRITISH DESMIDIE^. 



hence they are somewhat trapezoid ; their basal spines are nearly straight, the 

 rest are curved. 



This species is sufficiently marked by its solitary basal spines. 



Length of frond, not including spines, -^^ of an inch ; including spines, 

 gJ-g- ; breadth, not including spines, ^-Ir 5 including spines, -^^ ; breadth at 

 constriction y^^-g-. 



Length of frond of var. tcncinatnm, not including spines, -j^ ; including 

 spines, -3^ ; greatest breadth, not including spines, ^iy ; including spines, -^^ ; 

 breadth at end yaVo 5 hreadth at constriction -a^Vg. 



Tab. XIX. fig. 3. a. front view of typal form ; b. empty frond ; c. end 

 view ; d. front view of var. uncinatum ; e. empty frond ; f. end view. 



6. X? octocorne (Ehr.) ; segments much compressed, without a cen- 

 tral protuberance, trapezoid, each angle terminated by one or two 

 spines. 



a. One spine at each angle. 



/8. major ; larger ; two or more spines at each angle. 

 Arthrodesmus octocornis, Ehr. In/us. p. 152 (1838). Hassall, Brit. Alg. 



p. 357. Brebisson, in lit. 

 Micrasterias octoco?'nis, Meneghini, Si/nop. Des?nid. inLinncea 1840, p. 216. 

 Cosmarium jpalmatum, Brebisson, according to Menegh. in Synop. Besmid. 



(1840). 

 Staurastrumi octocorne, Ralfs, in Annals of Nat. Hist. v. 15. p. 159. t. 12. 



f. 3 (1845) ; Trans, of Bot. Soc. of Edinburgh, v. 2. p. 145. t. 15. 

 Xanthidium octocorne, Ehr. Meteorp. t. 1. f. 22. 

 Euastriim octocorne, Kiitzing, Phy. Germ. p. 134 (1845). 



a. Dolgelley and Penzance, /. R. Yate near Bristol, Mr. Broome. Amble- 

 side, Westmoreland, Mr. Sidebotham. Pulborough, Sussex ; Reigate, Surrey ; 

 and Weston Bogs near Southampton, Mr. Jenner. 

 /3. Dolgelley, /. R. 



a. Germany, Ehrenberg and Kiltzing. Falaise, Brebisson. Worden's 

 Pond, Rhode Island, Bailey. 



Frond minute, deeply constricted at the middle ; segments trapezoid, each 

 angle terminated by one or more straight spines, the intervals between the 

 angles concave. 



The variety /3, which I have gathered mixed with the more common form, 

 is usually larger, and its appearance, on account of the geminate spines, differs 

 so much from a. that Mr. Jenner regards it as another species ; but I believe 

 that no dependence can be placed on the number of spines as a specific 

 distinction, for in specimens which I sent to Mr. Jenner, he discovered one 

 frond whose lateral spines were single and end ones geminate, and other fronds 

 with three spines on each angle of one segment and two on each angle of the 

 other ; some again with the lateral spines geminate and the end spines in 

 threes. I have, in one instance, seen the spines double on one segment and 

 single on the other. 



The proper position of this plant is so doubtful, that different observers 

 have referred it to widely different genera. Ehrenberg placed it first in Ar- 

 throdesmus and afterwards in Xanthidium : Brebisson considered it a Cos- 



