Mr. G. H. K. Thwaites on the Gonidia of Lichens. 221. 
the gelatinous substance of the plant, and most thickly towards 
the periphery of the cylindrical branches of the fronds. Each 
cell is found, upon a careful inspection, to be surrounded by its 
definite amount of gelatine, and to be situated at the extremity 
of an ultimate ramification of the numerous somewhat anasto-. 
mosing filaments which pervade the whole mass of the plant 
(Pl. VIII. A. fig. 2). The genus Paulia, Fée, a species of which 
(Paulia perforata, Mont. MSS.) has, at the request of Mr. Berke- 
ley, been kindly sent for my inspection by Dr. Montagne, pos- 
sesses an internal structure precisely similar in character to that 
of Synalissa. The asci of Synalissa vulgaris contain numerous 
perfectly spherical sporidia: I could not detect any apothecia in 
Dr. Montagne’s specimen of Paulia. The genus Lichina is im- 
mediately allied to Stigonema (Kphebe, Fr.), and the whole struc- 
ture is very different from that of Paula, as I have ascertained 
from the examination of freshly-gathered specimens of the former 
recently sent me by Prof. Harvey. 
Whilst writing on this subject, | may mention another ver 
interesting plant, which, im the texture of its frond and character 
of its fructification, exhibits some analogy to Collema. I allude 
to Mastodia tessellata, Flor. Ant., for a sight of specimens of 
which | have been indebted to the kindness of Professor Harvey 
and Mr. Berkeley. The essential structure of this plant is re- 
presented by the genus Ulva (especially Ulva crispa), but it pos- 
sesses apothecia containing asci, though the latter organs appear 
to have escaped the observation of the excellent botanists who 
described the plant, owing to the sporidia so soon becoming free. 
We have thus then offered to our view plants which, judging 
from their external appearance alone, would be arranged together 
im one undivided group, and even in some cases in the same 
genus, exhibiting nevertheless totally different types of structure. 
They are as follows :— ‘ 
1. The Lichens proper ; 
2. Collema, Leptogium, &c. ; 
3. Synalissa and Paulia ; 
4. Mastodia ; 
represented respectively, as regards their essential fundamental 
structure, by the genera Pleurococcus, Nostoc, Coccochloris and 
Ulva (U. crispa), which are usually placed very near together in 
a natural arrangement; but the circumstance of their each im- 
pressing a character, upon being a bond of union, as it were, to 
plants higher in the scale of vegetation, will doubtless, if well 
considered, furnish a key to the proper arrangement of species 
closely allied to and of equally low development with them. 
It is highly interesting to observe in these lower plants a 
typical character of essential structure binding together nume- 
