168 
On the History, Histological Structure, 
seen several, sometimes singly or scattered irregularly on all sides 
of the walls of the tube, rarely in a continuous series, as in 
specimen figured at b on Plate XXXII. These markings are irregular 
in outline, and are generally split up into irregular polygons by 
cracks which pass right through them. They are without doubt 
placed on the inner surface of the tube. This may be determined 
by careful focussing, and it is distinctly shown in such a section as 
that shown on Plate XXXII., Fig. a, where a tube is seen opposite 
the asterisk in longitudinal section, and the markings are on the 
inner surface of the wall of the tube. 
The walls of the large tubes are of considerable thickness. 
When highly magnified they exhibit a series of minute dots 
covering their surface. Mr. Archer, in the account of his exa- 
mination of a section of the fossil, says, “ the principal longitudinal 
tubes appeared on transverse section to have a wall concentrically 
stratified.”* I have most carefully searched for this appearance 
without being able to detect it. In some cases where the section 
of the tube is slightly oblique, I have noticed an indication of a 
concentric arrangement ; but this was due, I believe, to the 
different planes of minute dots coming into view as the object-glass 
was being moved towards the specimen. I do not, however, 
believe that so experienced and accurate an observer as Mr. Archer 
could have been misled in this way : the cautious way in which 
his opinion is expressed induces me to express the hope that he 
may again examine the specimen in the light of this criticism. 
The description of this fossil which I have given, and which 
has been of necessity so destructive of what has been said before 
regarding it, establishes that its tissues are of the kind known to 
botanists as “ tela contexta.” They consist of elongated cylindrical 
cells of two sizes, interwoven irregularly into a kind of felted mass. 
No other tissue of any kind exists in the structure of the fossil. 
This peculiar tissue is very limited in its occurrence in the vege- 
table kingdom as it at present exists, being confined to the three 
lower groups of the cellular cryptogams. It forms the cottony 
medullary layer in the foliaceous Lichens, the mycelium of the 
Fungi, and the substance of some Algae. No one would venture to 
suggest any relations with the two first Classes, and though 
Nemcdophycus is an anomalous Alga, it must be placed in this 
group of plants. We must remember that we are dealing with a 
portion of the vegetative structure only; the pseudo-exogenous 
appearance seems to indicate that that portion is an axis. 
The sea-weeds in which a structure corresponding to that in 
Nematophycus occurs, are the Siphonese , Dasydadese, and 
Valoniacese , orders of Chlorosperms, the class which without doubt 
occupies the lowest place among the Algae, because of the simple 
structure of the plants included in it. The three Orders which I 
* ‘ Quart. Journ. Micr. Sc.,’ vol. xi., p. 319. 
