22 



It is thus seen that il was possible to indicate live different forms of ff. Opuntia, under 

 the names already existing for definite species and varieties; but there remained still a small 

 number of plants among the collections, sufficiently unlike the plants already dealt with as to be 

 worthy of record. The joints of such plants resemble ivy leaves \ I have therefore founded on 

 them the form hederacea. 



I would once more strongly emphasize the fact that it is impossible to regard these 

 forms as anything more than indications of the several directions of variation, exhibited by the 

 joint. This is especially well shewn in //. Opuntia, where a plant is rarely found in which all 

 the joints are sufficiently alike to allow of it being placed under one form only and in many 

 plants there are joints which represent as many as three different forms. 



ff. Opuntia is confined to the tropics, where it is common in all oceans. 



5. ff. gracilis Harv. 



Halimeda gracilis J. Agardh, Till Alg. Syst. VIII. p. 82. 



Halimeda gracilis De Toni, Syll. Alg. vol. I 1889. p. 521. 



Halimeda cuneata Kützing, Tab. Phyc. vol. VII. 1857. p. 8. tab. 21. hg. III. 



Halimeda cuneata Kützing in De Toni, Syll. Alg. vol. I. 1889. p. 526. 



Halimeda laxa Barton, Journ. Linn. Soc. Bot. vol. XXXIV. 1900. p. 479. PI. 18. figs. 1 — 3. 



Halimeda Opuntia var. intermedia Grunow in Herb. 



Halimeda Opuntia var. crassiuscula Grunow in Herb. 



Hab. Atlantic. — St. Thomas, Challenger ! — Fernando Neronha, Challenger ! — 



Ixdic. — Bentotta, Ceylon, Ferguson ! in Herb. Grunow sub nom. H. opuntia var. intermedia 



Grun. — Ceylon, Harvey\, n°. 72. Ferguson\, n"\ 88, 160. Dr. Oudaatje'. — 

 Siboga-Expedition. Postillon Islands, 30 m. ! — Aru Islands I — 



Pacific. — Honolulu, Grunotvl, in Herb. Grunow sub nom. H. opuntia var. crassiuscula Grun. 

 Tintaga, Funafuti, David\ n ns . A. 31. A. 34. — Torres Straits, " Challenger'' '! 

 "Kamtschatka", Tilesius in Herb. Kützing! — 



Plants varying in length, reaching to more than 40 cm., thickly calcified below, less so 

 above in long plants ; branched in one plane. Branches short and numerous, or long and few. 



Joints cuneate to subcylindrical ; not ribbed, margin of branched joints slightly nndulating, 

 V 2 — 9 mm. long, iY. — 11 mm. broad, 1 — 2 mm. thick. 



Filaments of central strand fuse in pairs, fusion complete, single fused filament branching 

 later trichotomously (fig. 30). 



Peripheral cells 30 — 45 u. across, surface view; 50 — 75 u. long. Side walls usually con- 

 nected for about 1 / 12 of length. Cells directly in contact or lying in a matrix of cellulose, ren- 

 dered visible by decalcification. 



forma lypica. 



Joints more or less cuneate, walls of peripheral cells thin, directly in contact (fig. 28). 



forma laxa. 



ff. laxa Bart. 

 Plant long, straggling, branches few, distant. Joints mostly subcylindrical, less calcified 



