148 ~THE PALEONTOLOGY OF MINNESOTA. 
[Pachydictya. 
Perhaps, the chief ones of the characters about to be mentioned, are those that have 
resulted in the presence and early development of interzocecial spaces. These begin, 
generally at any rate, the same as in the Cystodictyonide and the bifoliate Fistulipor- 
ide (Meekopora Ulrich) at the basal (mesial) plate, causing the primitive cell of the 
zocecial tubes to be in part separated from its neighbors, and to have a shape quite 
different from that of the Rhinidictyonide. Indeed, the resemblance to the semi- 
cordate cell so prevalent among the Cystodictyonida, is often very striking. (See 
plate IX, figs 8 and 15.) A common condition is when a small triangular interspace 
has been cut off from each of two diagonally opposite corners of the primitive cell. 
These interspaces increase in size and form shallow vesicles as growth proceeds, and 
as soon as the tubes have assumed an erect position, they are completely isolated by 
the superimposed vesicles. At the same time their walls become more or less 
thickened and ring-like, and, from now on to the surface, the zocecial investment 
remains, almost invariably, clearly distinguishable from the interspaces proper, the 
sharpness of definition between them being in most cases even increased after the 
interspaces have been filled with the usual solid deposit. These changes in the 
zocecial structure are shown in the various figures on plate IX. 
In Phyllodictya and Trigonodictya, as well as in some of the small species of Sec- 
tion b of Pachydictya, we have no positive evidence of the development of the inter- 
stitial vesicles until after the zocecia have left the mesial plate. In these, therefore, 
the basal portions of adjoining zocecia are in contact, and in that respect the same 
as in Rhinidictya. ‘To what extent this fact depreciates the value of the character 
of the partial separation mentioned in the preceding paragraph, Tam not prepared 
to say. Perhaps it finds an explanation in this that the character, or rather the 
peculiar shape of zocecium to which the early presence of interstitial vesicles is due, 
and which is so characteristic of Devonian and Carboniferous bifoliate Bryozoa, had 
not in those earlier times become fully established. k 
A remarkable agreement of structure is presented by certain forms of Pachydic- 
tya (Section a) with the Carboniferous fistuliporoid genus Meekopora (e. g. M. clausa 
Ulrich). That there exists real or ancestral affinity between them | doubt, yet, if 
there is none, the similarity between them is all the more curious. Nor does it seem 
likely that the relations with the Cystodictyonide are any closer. Still, it cannot be 
denied that the evidence at hand points to a relationship with those families on the 
one side and the Rhinidictyonide on the other.* 
*A point of general interest presents itself here. As is well known, Nicholson and perhaps the majority of European 
paleontologists regard Fistulipora and its allies as belonging to the Aleyonaria group of corals. Now, if we will take the 
various species of Pachydictya, starting with the small forms comprised in Section )b, which everyone concedes to be 
unequivocal Bryozoa, and going through to such forms of Section @ as have the vesicular interstitial tissue well devel- 
oped, we establish a chain of evidence tending very strongly to prove their view wrong. The lunarium only is lacking 
to make the chain complete, but, as is well known, that feature is not restricted to the Fistuliporidw, Indeed, it is as 
well, if not better, developed in such universally conceded Bryozoa as the Cystodictyonide and Ceramoporide. But this 
is onlyZone of many chains that IT would very willingly publish if it were not for the time consumed in writing them up. 
