nemathecia, developed at both surfaces, and containing minute crnciaie 

 tetraspores, hidden among short, vertical fibres. The colour is probably a 

 full, dark brovi'nish-red, but in all our specimens has considerably faded, 

 and partly changed into dull-green. The substance is extremely hard and 

 rigid, and the plant shows no tendency to adhere to paper in drying. 



A very curious and rare Alga, whose peculiarly sponge-like 

 structure is but iuiperfectly given in our rudely executed figure, 

 which otherwise tolerably represents one of the larger and more 

 divided specimens in the Dublin herbarium. The mode of evo- 

 lution of the frond has yet to be ascertained. Judging by the 

 few specimens I have seen, and which are in different stages of 

 growth, I am disposed to think that the frond at an early stage 

 is solid, and perhaps smooth, but soon becomes covered over 

 with slender, anastomosing fibrils, which extend chiefly laterally, 

 and form the flattened, spongy lamina. Very old fronds produce 

 numerous small, flabeUiform or forked leaflets on the surface of 

 the spongy frond, and in these, after the figure had been com- 

 pleted, I detected tetraHpores, lodged in discoid nemathecia. No 

 other fructification has yet been observed. 



I am indebted to Dr. Bonder for a fragment of Preiss's ori- 

 ginal specimen, and to my often-mentioned and liberal friend 

 George Clifton, for the specimen here drawn, and others in va- 

 rious states. All bear the marks of long exposure to the 

 weather, and are much faded. 



As the cystocarpic fruit of Thamnodonimn is still unknown, 

 the exact affinities of the genus cannot be determined, but the 

 structure of the frond is so similar to that of the denser genera 

 of Gelidiacece, particularly of the group Chcetangiece, that I have 

 little hesitation in associating it with that family. At any rate 

 it is far removed from Polyphacum, with which Agardh placed 

 the species known to him. 



Fig. 1. Tii.\MNocLONiUM FLABELLiFORME, — the natural size. 2. Transverse 

 slice through one of the fibres of the spongy network, showing two axes, 

 sunk in a common cellular substance, and which would probably be resolved 

 into two fibres, the cellular matrix disappearing ? — magntjied. 



