No. 3] REMARKS ON NORTHERN LITHOTHAMNIA. 91 



of the preceding form, but they are usually smaller, 120 — 180 //• 

 in diameter. In f. circumscripta the conceptacles are situated very 

 little below the surface of the frond, or less deeply than in the 

 preceding forms, but they are more densely crowded, usually 

 forming a zone sharply defined. The central portion of the roof 

 at first becomes decorticated, by and by forming a very shallow 

 hole, 100 — 150 fi in diameter, the bottom of which forms a part 

 of the roof and is intersected with 10—20 or up to 30 muciferous 

 canals. After maturity this central portion first falls to decay, 

 and at the same time the surrounding parts of the roofs become 

 lighter and gradually disappear, frequently — owing to the con- 

 ceptacles densely crowded — leaving hexagonal holes, 200 — 300 

 or up to 350 /j. in diameter, the latter representing the emptied 

 conceptacles which look like a mesh-work with thin walls. Also 

 the greater part of the said intermediate walls falls to decay. 

 The scars are effaced by a new thickening layer of the frond 

 or by new local formations, and when the scars are nearly 

 replenished the surface often appears areolate. Such areolating is 

 sometimes to be seen also before the conceptacles are fully deve- 

 loped. In the form coalescens the conceptacles mainly resemble 

 those of the latter, but they are mostly less crowded. — The 

 sporangia in this species are frequently bisporic, now and then 

 tetrasporic, particularly in f. testacea, occasionally irregularly three- 

 or four-parted. Cp. above. They are rather varying in size, and 

 smallest in f. testacea, judging from a few measures, 90—120 (i 

 long and 25 — 50 jj. broad. In the other forms they are 100 — 300 ,a 

 or even up to 390 /* long and 50 — 100 \i or up to 150//- broad. 

 As to the synonymy, L. scabriusculum 1. c. probably repre- 

 sents a young and somewhat stunted form of the species in que- 

 stion. At any rate it will scarcely prove to be an independent 

 species. Besides, I now consider the Norwegian specimen of L. 

 evanescens mentioned 1. c. as representing a peculiar f. circum- 

 scripta. This is perhaps also the case with the typical specimen 

 from America. I have only seen the fragment pictured in Norw. 

 Lithoth. pi. 22, fig. 6. The decortication of the conceptacles seems 

 to take place in another way than in typical f. circumscripta, but 



