BRITISH DESMIDIEyE. 113 



Tab. XVIII. a. frond with endochrome ; b. side view ; c, d. fronds acquiring 

 new segments by division ; e. empty frond ; /. transverse view ; y. sporangium 

 with empty segments of the conjugated fronds. 



** Sinnes subulate. 



2. X. aculeatum (Ehr.) ; spines subulate, more or less scattered ; cen- 

 tral projections truncate, obscurely dentate. 



Xanthidium aculeatum, Ehr. Abh. d. Berl. Ak. (1833), p. 318 ; Infus. p. 147. 

 t. 10. f. 23. Meneghini, Synop. Desmid. in Lmncea 1840, p. 224. Ralfs, 

 in Annah of Nat. Hist. v. 14. p. 467. t. 12. f. 2 ; Trans, of Bot. Soc. 

 of Edinburgh, v. 2. p. 1 55. t. 1/. Jenner, Fl. ofTunbridge Wells, p. 194. 

 Hassall, Brit. Alg. p. 360. 



Weston Bogs near Southampton ; Ashdovra Forest, and Tunbridge Wells, 

 Mr. Jenner. Penzance, J. R. Yate near Bristol, Mr. Thwaites. 



Germany, Ehrenberg. 



Frond deeply constricted at the middle, so as to form a Hnear notch on each 

 side ; segments somewhat reniform, tvdce as broad as long ; spines subulate, 

 short, marginal and scattered. The central projections, as seen in end or 

 lateral views, are cylindrical, truncate, and bordered by pearly granules which 

 give a dentate appearance, but less conspicuously than in Xanthidium armatum. 



Xanthidium aculeatum varies in the arrangement and number of its spines. 

 The Bristol specimens have a far larger central projection than the other speci- 

 mens which I have seen. 



This species is distinguished from all the following by having its spines more 

 or less scattered. 



Length of frond, not including spines, from g-i^ to -g-fy of an inch ; including 

 spines, from g^-j to 3-^-3 ; breadth, not including spines, from 3-i^ to 3^^ ; in- 

 cluding spines, from -^^ to -^-^ ; breadth at constriction from y-g^g ^^ ttVt- 



Tab. XIX. fig. 1 . a,e, g. fronds with endochrome ; b, empty frond \ c,h. side 

 views; </,_/. end ^'iews. 



3. X. Brehissonii ; spines subulate, marginal, geminate; central 



projection somewhat truncate and margined with pearly granules. 



/3. varians ; segments broader and more irregular ; spines some- 

 what irregular and unequal. 



Binatella actdeata, Brebisson, Alg. Fal. p. 58. t. 8 (1835). 



Cosmarium aculeatum, Meneghini, Synop. Desmid. in Einncea 1840, p. 218 



(ui part). 

 Euastrum, No. 1 1, Bailey, Amer. Bacil. in Amer. Journ. of Science and Arts, 



V. 41. t. l.f. 13 (1841). 

 Xanthidium bisenarium, Ehrenberg, Verb, und Einfluss des mikrosk. Lebens 



in Sad- %md Nor d- Amer. (1843). 

 Xanthidium aculeatum, Brebisson, in lit. cum icone (1846). 



a. Piltdown Common, Mr. Jenner. Bristol, Mr. Thwaites. Penzance, J. R. 

 l3. Piltdown Common, Sussex, 3Ir. Jenner. Trewellard near Penzance, J. R. 



Falaise, Brebisson. West Point, New York, Bailey. 



