McLean . — On the Fossil Gcmcs Sporocarpon. 75 
The external envelope is formed, in the young state, of cylindrical 
cells, closely contiguous in the tangential direction and arranged with their 
long axes radially. There is only one layer, and the outer extremities of 
all are more or less abruptly acuminate. Although they are almost truly 
circular in cross-section there are no interspaces between the cells, for the 
spaces between the lines of actual contact are filled in by extra thickening 
of the walls, and the cells are slightly broader at the distal ends. In most 
specimens, and in those the best preserved, the outer pointed extremities of 
these cells or vesicles bear small dark papillae which may be seen to have 
finely tufted points, if a high magnification be used. By comparison with 
Traquairia , in which all the radial spines bear fibroid processes, which are 
clearly hollow, and serve to connect the interior lumen of the spine with 
the outside, it seems legitimate to conclude that these minute papillae are 
of similar nature. 
The centripetal ends — the bases — of the cells abut upon the wall of 
the sphere, which is smooth and often fairly thick. The average length 
of the cells of the envelope is only about one quarter the diameter of the 
spherical interior space. The question of proximal openings into the interior 
cavity has not been at all easy to decide in this species. In some of the 
others, and in Traquairia , there can be no doubt that such do exist, a fact 
which strengthens greatly the unicellular view of the organism. Extremely 
thin, perfectly radial sections are scarcely possible, and it is generally in 
a somewhat thick tangential section, giving a hemispherical view, that the 
structure of the sphere wall is best seen. Those that have been available 
to me have shown no sign of any large openings, but in several specimens 
the sphere wall and the radial walls of each cell for a short distance up 
from the base can be seen to be covered with a very fine punctate marking, 
like the ‘dots’ on diatom frustules and just about as minute. Quite 
possibly this may have been a porose sieve-area which permitted junction 
of the protoplasm between the cells, and with the interior cavity. 
That the cells of the envelope did contain living matter it is impossible 
to doubt, for they show a marked power of radial elongation. In older 
specimens many of the cells will be found lengthened into spines, three or 
four times as long as the original cells. These spines are rather obtuse, 
never acuminate as in the embryonic condition, so that it is evident that in 
growing the pointed apical portion of the cell must have shared in the 
extension of surface, and not have been merely pushed outwards from 
within by the intercalary elongation of the thinner radial walls. The whole 
mature spine, above the level of the surrounding cells, tapers gradually, and 
is covered with the minute nipples spoken of before, whose porose nature 
can sometimes be clearly seen. Some at least of these must have been 
added during the secondary growth of the cell. No instance of all the 
cells of an individual becoming elongated has come to my notice. Usually 
