ARTHRODKSMUS. ( J 1 



1893, p. 559; Eoy A. Hiss. Scott. Desm. 1S94-, p. 10; Nordst. Index Desm. 

 1896, p. 145 ; W. & G. S. West, Alar. S. England, 1S97, p. 490; Alja-fi. 

 Yorks. 1901, p. J09; Korg. Fresh w. Alo-. Faeroes, 1901, p. 228 ; W. & 

 G. S. West, Alo-. N. Ireland, 1902, p. 58; Him, Desm. Finland, 19U3, 

 p. 5 ; W. & ijK S. West, Scott, Freshw. Plankton, I, 1903, p. 527 ; lior>v. 

 Alg-. erst. Regnell. Exped. II. Desmid. 1903 p. 103; W. & G. S. NVest, 

 Fresh w. Alg. Orkneys and Shetlands, 1905, p. 23; Further Onitrib. 

 Plankton Scott. Lochs, 1905, p. 485 ; Coinp. Study Plankton Irish Lakes, 

 1906, p. 85 ; Jtori^e, Ueitrag-e Alg. Schwedeii, 1906, p. 49 ; Larsen, 

 Ferskvandsalg. Vest-Gronl. ' 1907, p. 324; W. & G. S. West, Brit. 

 Freshw. Phytoplankton, etc., 1909, p. 181 ; Phytoplanktoii Engl. Lake 

 District, 1909, p. 138. 



-uHi retusuni Kiitz. Phyc. germ. 1845, p. 136. 



Cosmarium Incus De Bary, Conj. 1858, p. 72. 



C. retusum (Kiitz.) Rossmann. in Jahresber. d. Wetterau. Ges. f. die 

 gesammte Naturkunde zu Hauau, 1861, p. 7. 



Arthrodesmus Incus b. Lund. Desm. Suec. 1871, p. 55. 



Duly >ni il'nnn erectum Keinsch, Algeufl. Frank. 1867, p. 157 [in part]. 



Ai-throctesmus Incus var. diver gens Cooke, Brit. Desm. 1887, p. 136; West, 

 Alg. X. Wales, 1890, p. 291. 



A. Incus forma typica Heimerl, Desm. alp. 1891, p. 603, t. 5, f. 17 ; Liitkem. 

 Desm. MiUstattersees, 1900, p. 13. 



Cells rather small, a little longer than broad 

 (without the spines), deeply constricted, sinus widely 

 open and sometimes submamillate at the extremity ; 

 semicells obversely triangular or obversely subtrapezi- 

 form-triangular, apex straight, rarely slightly concave, 

 sides very faintly convex, each ano-le only slightly 



/ t/ O */ O *J 



rounded and furnished with an elongated, stout, 

 diverging spine. Vertical view elliptic, each pole 

 furnished with a long, stout spine. Cell- wall smooth. 

 Chloroplasts axile, one in each semicell, sometimes 

 containing a conspicuous pyrenoid. 



Zygospore not definitely known, but probably 

 furnished with simple spines. 



Length without spines 21-27 p,, with spines 4-1 

 oOjut; breadth without spines 18-i2o p., with spines 

 50-71 p, ; length of spines 20-28 p, ; breadth of isthmus 

 7-9 p ; thickness 10-11*5 p. 



ENGLAND.- -Cumberland ! Westmoreland! Surrey ! 

 Cornwall! In the plankton of Buttermere, Enner- 

 dale Water, and Bassenthwaite Water, Cumberland! 

 In the plankton of Easdale and Coclale Tarns, West- 

 moreland ! 



WALES.- -In the plankton of several of the lakes of 

 Carnarvonshire ! 



