90 BRITISH DESMIDIACE.E. 



tf Angles of semicells furnished with very short spines, 



often merely mucronate. 

 + Apex of semicell convex. 



Cells small, vertical view rhomboid. 



4. A., crassus. 

 Cells minute, vertical view elliptic. 



5. A. controversies. 



++ Apex of semicells retuse, making the semicells 

 triangular-lunate. 



6. A. lihiwiis. 

 Semicells subrectangular, sinus closed and linear. 



7. A. Bulnheimii. 

 *** Semicells elliptic or narrowlv elliptic. 



J- ^ X 



t Spines rather short and incurved, sinus closed at the 

 apex and opening outwards. 



8. A. convergent. 



ft Spines longer and disposed horizontally, sinus open 

 from the extreme apex. 



9. A. siibulatus. 



SECTION B. Semicells with two spines on each side, one 

 attached to each of the two lateral angles. [Octacanthium Hansg.] 

 Semicells rectangular with retuse sides, spines fairly long. 



10. A. octocornis. 

 Semicells hexagonal depressed, spines short. 



11. A. bifid us. 

 SECTION C. Semicells with three spines on each side. 



Three spines in the same vertical plane. 



12. A. trispinatus. 

 Three spines not in the same plane ; one lateral and two 



apical, the latter forming a pair in a horizontal plane. 



13. A. temiissiiuus. 



1. Arthrodesmus Incus (Breb.) Hass. 

 (PL CXIII, figs. 13-15.) 



Binatella Incus Breb. Alg. Falaise, 1835. p. 269. 



Heterocarpella Incus Breb. in Cheval. microscop. et usage, 1839, p. 272. 



Staurastrum Incus (Breb.) Menegli. Synops. Desm. 1840, p. 228; Kalfs 

 in Ann. Mag. Nat. Hist. 1845, xv, p. 158, 1. 12, f . 2 ; Jacobs. Desm. Danem. 

 187(5, p. 204 [in part]. 



Arthrodesmus Incus (Breb.) Hass. Brit. Fresliw. Alg. 1845, p. 357, t. 85, 

 f. 10; Ealfs, Brit. Desm. 1848, p. 118, t. 20, f. 4 a-d; Ardi. in Pi-itch. 

 Infus. 1861, p. 737 [in part] ; Babenh. Flor. Europ. Alg. Ill, 1868, 

 p. 226 [in part] ; Nordst. Desm. Brasil. 1870, p. 231 ; Kirchn. Alg. 

 Schles. 1878, p. 156; Wolle Desm. U.S. 1884, p. 1)7, t. 24, f. 5 (?)-, 

 Boldt, Siber. Cblor. 1885, p. 109; Cooke, Brit, Desm. 1SS7, p. 135, t. 47, 

 f. 4 [in part] ; Haiisg. Prodr. Algenfl. lii)lim. 1888, p. 202 ; Boldt, 

 Desmid. Gronland, 18S8, p. 30; Nordst, FreshAv. Alg. N. Z<-al. 18-s, 

 p. 45; De Toni, Syll. Aly. 1 }si), p. 1O57: West. Al-\ AY. Iivland, 1892, 

 p. 168; Alg. Engl. Lake Distr. 1892, p. 730; Liitkem. Desm. Attersees, 



