XANTHIDIUM. 51 



*** Semicells elliptic-hexagonal, lateral spines in a vertical 

 plane, central area absent or small, sometimes 

 scrobiculated. 



4. X. controversum. 

 />. Semicells with two pairs of spines on each side. 



* Cells small ; semicells trapeziform with retuse (or 

 straight) sides and apex. 



5. X. Smitkii. 



** Cells of medium size ; semicells more or less elliptic- 

 hexagonal. 



6. X. antilopxum. 



c. Semicells with two pairs of spines and one single spine on 



each side. 



7. X. cristatum. 



d. Semicells with three pairs of spines on each side. 



8. X.fasciculatum. 



e. Semicells with about 8-10 spines on each side, in pairs or 



more or less irregularly scattered. 



a/ 



Spines mostly in pairs. 



9. X. Brebissonii. 

 ** Spines irregularly scattered. 



10. X. aculeatum. 



SECTION C. Min-acauthum Turner. Cells small or verv small, 



*' 



with very minute simple spines. 



11. X. var labile. 



12. X. Robinsonianum. 



13. X. Orcadense. 



14. X. apicullferiun. 



15. X. concinnum. 



1. Xanthidium armatum (Breb.) Rabenli. 

 (PI. CIV, figs. 1-5.) 



Cosmarium armatum Breb. in Menegii. Synops. Desm. 1840, p. 218. 



Enastt'iim armatrnm Kiitz. Phyc. germ. 1845, p. 137. 



Xtrnthidium furcation Ealfs in Ann. Mag. Xat. Hist, xiv, 1845, p. 466, 



t, 12, f. 1. 

 X n thulium annation (Breb.) Eabeiih. Deutschl. Kryptogamenfl. Bd. II, 



Algeii, Leipsic, 1847, p. 55 ; Ealfs, Brit. Desm. 1848, p. 112, 1. 18 ; Arch. 



in Priteh. Infus. 1861, p. 735, t. 1, f. 27-28; Eabenh. Flor. Europ. 



Algar. Ill, 1868, p. 222 ; Lund. Desm. Snec. 1871, p. 75, t. 5, f. 4 ; Arch. 



in Q. J. Micr. Sci. xiii, 1873, p. 37 ; Kirchn. Alg. Schles. 1878, p. 154 ; 



Wolle, Desm. U. S. 1884, p. 92, t. 21, f. 2-4 [figures poor] ; Cooke, Brit. 



Desni. 1887, p. 129, t. 45, f. 1 [figures very poor] ; Hansg. Prodr. 



Algenfl. Bohm. 1888, p. 191, f. 114; De Toiii, Syll. Alg. 1889, p. 916; 



West, Alg. X. Wales, 1890, p. 291 ; Alg. W. Ireland, 1892, p. 164 ; Alg. 



Eiigl. Lake District, 1892, p. 730 ; Eoy & Biss. Scott, Desm. 1893, p. 244 ; 



Xordst. Index Desm. 1896, p. 51 ; W. & GK S. West, Alg. S. England, 



1897, p. 484 ; Schmidle, Lappmark Siisswasseralgen, 1898, p. 21 ; W. & 



G. S. West, Alga-fl. Yorks. 1900, p. 67 ; Borg. Freshw. Alg. Faeroes, 



