284 DR. LILY BATTEN ON 
12. Pericentral siphons spirally curved.......... eee e nn nn n P, atro-rubescens, 
Not so..........,.......... ehh hm hr een 13 
13. Maximum height of plant 4 inch ..... E e P. obscura. 
Not so....... . tee e e TE ev 14 
14. Plant corticate throughout ee e eee T noces P. fruticulosa. 
Not so. TE TEE TE TE T 15 
15. Colour bright brick- red when fresh ee eee es P, subulifera. 
Not. TOP nn TENE TEES 16 
16. Central siphon small. TEE P. simulans, 
Central siphon occupying about half the radius of thallus in 
transverse section..... TN P. nigrescens. 
17, Filaments decumbent ; plant not more than 4 inch high eese P, obscura, 
Not s0........, ren TE Leen eee eee 18 
18. Usually parasitic on Ascophyllum nodosum, and containing con- 
spieuous central siphon .................. ka P. fastigiata. 
Not so..............,.............. TN 19 
19. Zone of corticating cells present outside the primary - siphons at 
extreme base of main axis .......... ren P. nigrescens. 
Axis consisting solely of about 20 pericentral siphons, and one 
central, ecorticate throughout. ...,,......,.....,........... P. opaca. 
20, Plant ecorticate ........ TE E 21 
Plant corticate ,..,......... 2 . DM 28 
21. Siphons spirally curved in principal branches TEE TENE 25 
Not s0..,,,,,...,...,... e. TE es 22 
22, Plants gelatinous, oceurring in dufis about | inch in height ...... P. macrocarpa, 
Not so....., TE TE e. 24 
23. Articulations one or 1} times as s long as broad,...,............ © D. spiralis. 
Articulations 6-8 times as long as broad .............., ee P. fibrata. 
24. Colour deep full red .,..,,....,............ tee ee P. urceolata, 
Not so ,........, I1" eee ue TE 25 
25. Articulations 4-6 times s as long as broad ............... Dour 26 
Not s0........ Chae ee ene sense esesuess I 27 
26. Ramuli dichotomously branched, desc ern ren P. fibrata. 
Ramuli alternate or unilateral............ vers. TE P. insidiosa. 
27. Plant greyish in colour. Articulations 3 or 4 times as long as 
broad ...,....,... TE TO TEE EPOD P. insidiosa. 
Colour reddish-brown. Articulations 2 or 3 times as long as broad. P, rhunensis. 
28. Plant pale straw-coloured or brownish ..,........ ........ ees P. fibrillosa. 
Not so, T I ee ess tee een ete ete 29 
29. Ramuli tapering at base and apex ......... MEM ess DP. elongata. 
Not so..............,, nn nn I 30 
30. Plants tufted from the buse 0 ee a ren 31 
Having conspicuous main axis.............. TOUR . 32 
31. Ramuli dichotomous............. ee ME P. fibrata. 
Ramuli alternate ,,....,,,,.,............ esee esses sns P. spinulosa. 
32. Axils of lower branches markedly obtuse.......... EMEN . P. elongella. 
Axils of lower branches acute. .......,.............. ........ P. violacea. 
