162 THE PALEONTOLOGY OF MINNESOTA. 



LPtilodictyonldm. 



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 Phamopora, 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 Phcenopora; 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 Ptcenopora 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 Phcenopora and then Ptilodictya were evolved. In the 

 development of the former, the connecting channel between the apertures was cnt 

 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 Phcenopora 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 zorecial tubes of early Phcenopora 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 obtakied through all the most 

 typical members of the family. 



The systematic position of Stidoporella is undoubtedly near that of Intrapora, 

 Hall, Tmniodictya, 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 

 Ptilodictyonida 1 , 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 Ptilodictyonida; 

 would surely be a more compact and obviously characterized group if they were re- 



