76 McLean . — On the Fossil Genus Sporocarpon. 
only three or four are present in any given section, often grouped closely 
together, which is a clear distinguishing character between this species and 
N. elegans , where all the peripheral cells develop, though not all at the 
same time. 
This elongation demonstrates clearly that protoplasm lived in the cells 
of the envelope ; and, secondly, that the walls, even the thickened external 
walls, were not merely skeletal as in the Radiolaria, but were formed of 
organic material capable of extension. It is probable that the secondary 
growth of the cells was conditioned by the demand for an increased number 
of pores of communication in the adult reproductive organism, since it is 
through these atria that absorption would be chiefly carried on. 
The whole organism bears a certain resemblance to Volvox , though 
with obvious differences. The formation of reproductive cells in the 
interior cavity, unless the protoplasm migrated there centripetally, hardly 
seems compatible with colonial nature. Most species show clearly that the 
various spaces of the organism were in open communication, and the 
localization of reproduction in the large interior cavity suggests strongly 
that this was a living part of the organism, not merely a mucilaginous 
medulla. No segmentation of the contents has been noticed except at 
sporulation, so that all the evidence points to their being truly unicellular, 
or at least coenocytic organisms. The method of reproduction in all the 
species is essentially similar, so that one description will serve for all. 
Within the outer wall of the sphere comes a delicate ‘ capsule wall 1 
normally in close contact with the sphere wall, but often shrunk away 
from it. This may bear fine markings, and is sometimes apparently formed 
of two separable thicknesses. The capsule contains a closely packed mass 
of thin-walled vesicles (the so-called sporoids of my first account), frequently 
embedded in froth y periplasm. Commonly these sporoids are empty, but 
one frequently finds specimens in which each one, more or less, holds 
a small dark-walled spore with a sculptured coat. These last may be 
fungal, but their mode of occurrence leans me to the view that they are the 
real reproductive cells ; the sporoids being of the nature of mother-cells. 
There is no evidence to show how they were dispersed. 1 Diminutive or 
otherwise juvenile individuals have no sporoids in their capsules, spore 
formation only taking place at maturity. 
Very little can be said about the subsequent history of these spores. 
Truly embryonic stages in the development of the spheres have not 
been recognized with certainty, and I am inclined to think that they 
were not, until approaching maturity, of a suitable consistency for fos- 
sil ization. It is possible that N. Oidospora may be a young stage of 
1 Since the above was written I have received a very interesting drawing from Mrs. Scott | j 
(Fig. 9), in which a specimen of S. asteroides is shown (Scott Collection, 1788) apparently extruding 
the contents of its capsule through a dehiscence of the envelope, not by means of any definitely 1 
organized aperture. 
