o4 Dr. A. G. Nathorst—Silurian “ Plants” of Wales. 
The different manner in which the supposed “seaweeds,” described 
by Mr. Keeping, occur in the rock, harmonizes most _per- 
fectly with the opinion that they are worm trails. That this 
opinion is correct is finally put beyond doubt by Mr. Keeping’s 
statement, “that it is noteworthy that these structures usually 
project as convex bodies upon the under surfaces of the grit beds.” 
Or, to speak in other words, the markings referred to are casts of 
furrows or depressions, produced in the surface of the underlying beds 
when still soft. True plant fossils never occur in this way, but it is, on 
the other hand, quite clear that this is the mode of occurrence, in 
which trails of animals must present themselves. As far as I can 
understand, this single statement is thus quite sufficient to prove 
that the markings in question cannot have been plants. Buthotre- 
phis major, B. minor, Paleochorda tardifurcata, Nematolites Edwardsi, 
should consequently be erased from the lists of fossil plants. 
. As to Retiofucus Mr. Keeping certainly states, that “the vege- 
table nature of this structure cannot however be doubted,” but for 
what reasons the reader is not informed. I should however think, 
that every one has a right to claim that such a statement should be 
based on comparison with living plants, and until it has been shown 
that any relation between the structure of Retiofucus and some 
modern alga really exists, there is no reason why one should regard 
the former as being of organic origin. Not having examined the 
specimen myself, J cannot express any decided opinion as to its mode 
of formation, but I will not omit to mention, that I have seen a 
pretty analogous structure on the sea-beach between Mundesley 
and Cromer, when visiting England in 1872. The water of the sea, 
being very much troubled with mud, had given rise to a dark and 
dirty foam, and this, when left on the sandy shore at low water, 
produced, in bursting, a net-like structure, the meshes of which, 
corresponding to the air-bubbles of the foam, were formed by loamy 
ridges. This structure if hardened would have very much resembled 
the Retiofucus although it is highly probable that this might have 
been formed in some different way. 
Thus much as to the supposed plant-fossils described by Mr. 
Keeping. Before ending I should however like to givea hint to any 
one working with such doubtful bodies. First examine closely their 
manner of occurrence, and do not compare them with plants, if this 
examination has not shown that their being fossil-plants is really 
possible. 
As to Myrianites Lapworthiit, Mr. Keeping might be quite right in 
comparing it with the trail of a worm, although it is by no means 
impossible that it may have been produced by a crustacean. But 
why should one give names at all to such objects, since I have else- 
where shown that different animals might produce very similar trails, 
while the same animal under different conditions may give rise to 
very different ones ? 
