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 Cystodictyonidce and the bifoliate Fistulipor- 

 idce (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 Rhinidictyonidce. Indeed, the resemblance to the semi- 

 cordate cell so prevalent among the Cystodictyonidce, is often very striking. (See 

 plate IX, figs 8 and 13.) 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 zooecial 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 Phyllodidya and Trigonodidya, 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 Rhinididya. To what extent this fact depreciates the value of the character 

 of the partial separation mentioned in the preceding paragraph, I am 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. 



A remarkable agreement of structure is presented by certain forms of Pachydic- 

 tya (Section ) with the Carboniferous fistuliporoid genus Meekopora (e. g. M. clansa 

 Ulrich). That there exists real or ancestral affinity between them I 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 Cystodidyonidai 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 Rhinidictyonidce on the other.* 



* A point of general interest presents itself here. As is well known, Nicholson and perhaps the majority of European 

 paleontologists regard FistuUpora and its allies as belonging to the Alct/onarla group of corals. Now, if we will take the 

 various species of Pachydlctya, starting with the small forms comprised in Section ft, whioh everyone concedes to be 

 unequivocal Bryozoa. and going through to such forms of Section a as have the vesicular interstitial tissue well devel- 

 oped, we establish a chain of evidence tending very strongly to prove their view wrong. The luuarium only is lacking 

 to make the chain complete, but, as is well known, that feature is not restricted to the Fiftuliiuiridu-. Indeed, it is as 

 well, if not better, developed in such universally conceded Bryozoa as the CystodictyonitJke and CeramapuriOte. But this 

 is onlyjone of many chains that I would very willingly publish if it were not for the time consumed in writing them up. 



