162 THE PALEONTOLOGY OF MINNESOTA. 
| Ptilodictyo nidz. 
of not only their respective genera but of their entire family. In every case the 
generic features are fairly developed, indicating that the primal stock is yet to be 
discovered in previously deposited rocks. Still, in the three most typical members 
of the family, Ptilodictya, Escharopora and Phenopora, the resemblance between the 
primitive species of each is more evident than in the species that occur in deposits 
of later date. 
Indeed, in these early Bryozoa we often meet with species that combine, some- 
times to a very perplexing degree, characters which in latter times have attained 
the stability and importance of generic structures. Escharopora confluens and E. (?) 
limitaris are cases in point, since they have much to remind us of Phwnopora; not of 
the fully differentiated Upper Silurian forms of that genus, but of the Lower Silurian 
species which obviously had not yet attained the full expression of the generic char- 
acters. From the facts already available we are, I believe, justified in assuming either 
that Ptenopora and Escharopora are contemporaneous offshoots from a more primi- 
tive stock, with characters in general like those of E. confluens; or that Escharopora 
was the stock from which first Phenopora and then Ptilodictya were evolved. In the 
development of the former, the connecting channel between the apertures was cut 
off by the formation of a rim at their ends. The mere depression to which the chan- 
nel was thereby reduced, was next deepened, chiefly at the ends, thus giving rise to 
the two mesopores between the ends of the zocecial apertures. These are already 
well developed in Phenopora incipiens, but like all incipent characters are as yet a 
little unstable. The later development of the genus consisted principally in the 
greater separation of the longitudinal walls between which the primitive cells were 
arranged. This caused a shortening of the longitudinal inter-apertural spaces, with 
the result that the “two mesopores” were obliged to change their arrangement from 
the longitudinal to the transverse. 
The prostrate portion of the zoccial tubes of early Phenopora is very narrow 
and elongate, just as in the contemporaneous species of Escharopora, and the ten- 
dency to shorten and widen the primitive cell (already mentioned) exhibited in 
Middle and Upper Silurian times, seems to have obtained through all the most 
typical members of the family. 
The systematic position of Stictoporella is undoubtedly near that of Intrapora, 
Hall, Teniodictya, Stictotrypa, and Ptilotrypa, Ulrich. These five genera, it seems to 
me now, should be classed together, but whether they ought to be regarded as con 
stituting a distinct family by themselves, or had best be retained as members of the 
Ptilodictyonidw, the position assigned to them in my recent work on the Illinois 
Bryozoa, is a question that I am not yet prepared to solve. The Ptilodictyonide 
would surely be a more compact and obviously characterized group if they were re- 
