224 CERAMIACE.E. v. 



American specimens before me are clearly referable to Pt. serrata ; nevertheless, Sir 

 John Richardson collected on the Arctic coast both the true Pt. plumosa and Pt. 

 serrata. Finally, and this is rather puzzling, I have a specimen from Prince Ed- 

 ward's Island somewhat intermediate in character between both species, but having 

 the preponderance in favour of Pt. plumosa^ and yet being unlike any European 

 specimen of that species which I have seen. 



On the whole I am disposed, for the present, to adopt this species, considering it 

 more critically characterised in doubtful cases by its fructification than by its 

 ramification. 



Sect. 2. SmiUFOLiiE : Pinnce opposite, of similar nature, either both of the same length 

 or one longer, one shorter than the other ; the longer alternating on the rachis. 



6. Ptilota plumosa, Ag. ; frond piano-compressed, two-edged, decompound pin- 

 nate ; pinna3 and pinnute opposite, similar in form, but one frequently shorter than 

 the other, the shorter pinna simply pinnate, not altering ; the longer developing 

 into a branch, becoming compound ; ultimate pinnated ramuli, by suppression of 

 the opposing ramuli, frequently alternate ; pinnellse subulate, areolated with cells, 

 acute ; favellfe formed either on the pinnae or pinnelliB, involucrate, the branches of 

 the involucre subulate, entire ; tetraspores on marginal processes of the pinnellse. 

 J. Ag. Sp. Alg. 2, p. 96. Harv. Phyc. Brit. t. 80. Fucus ptlumosus, L. — Turn. 

 Hist. t. 60. {excl. var. ^.), E. Pot. t. 1308. 



Hab. Parasitical on the stems of Laminariw, &c. Prince William's Sound, North 

 West America, 1787, Mr. Menzies ! Arctic Sea Coast, Sir John Richardson ! Prince 

 Edward's Island, Dr. Jeans, (v. v.) 



Very like the preceding in aspect, and only to be known by the characters 

 detailed in the above descriptions and remarks. The different position of the 

 tetraspores is perhaps the most valid character. 



The true Pt. plumosa is of very rare occurrence in America. That which com- 

 monly passes under this name, in collections made to the North of Cape Cod, is Pt. 

 serrata ; that in those made to the south of that head-land is Pt. elegans. I have 

 examined and compared specimens of the true Pt. plumosa from the above localities, 

 with European individuals, and consider them specifically identical. 



^ 7. Ptilota elegans, Bonnem.; frond flaccid, filiform, terete, decompound-pinnate; 

 pinnaj and pinnulaj opposite, similar, both pinnately parted, the opposing one either 

 of equal size or one smaller than the other ; the younger pinnaj and all the ultimate 

 pumules articulate, composed of a single series of large, sub-quadrate cells, obtuse, 

 Imear (not attenuated); tetraspores terminating the ramuli, at length polysporous; 

 favelloB binate, naked, or sub-involucrate, on the pinnae. Bonnem. Hijdr. p. 22. 

 Kiltz. Phyc. p. 378. /. Ag. Sp. Alg. 2, p. 94. Pt. sericea, Harv. Phyc. Brit. t. 191. 



