214 THE PALEONTOLOGY OF MINNESOTA. 
(Trepostomata. 
I believe, be recognized at once, since it embraces every essential character of the 
genus. But it has others that are not possessed by any of the other species. Chief 
among these is the mesial ridge of the branches that brings to mind Semicosciniwm 
and Fenestralia of the Fenestellide. As shown, this ridge is the superficial extension 
ofa double median lamina constructed upon the same plan as in the Rhinidictyonide. 
Of course there is a difference in this that in the ordinary bifoliate Bryozoa the 
median lamina is horizontal while in the P. corticosa it is vertical to the plane 
of expansion. We find however precisely the latter condition in Goniocladia, 
Ethridge, jun., a Carboniferous genus with relations to the Cystodictyonide, and in 
Reticulipora, @Orbigny, a Cretaceous genus of the Cyclostomata. If we add to these 
peculiarities the general resemblance of the zocecial tubes of Phylloporina to those of 
ordinary Trepostemata, and the common possession of mesopores and acanthopores, 
we are confronted in P. corticosa by, to say the least,-an unexpected mixture of 
characters. These facts are not mentioned because they are believed to indicate 
true relationships among the diverse types enumerated. On the contrary, it is 
more than doubtful that any two of them belong to the same line of development. 
Still, there is more than mere coincidence«in the combination of characters seen in 
P. corticosa, and in the genealogical studies that will be in order sooner or later, 
they should be borne in mind. . 
Formation and locality.—Upper third of the Trenton shales, near Oxford Mills, Goodhue county, 
Minnesota, where it is abundant. A few specimens have been found also at St. Paul, in equivalent beds. 
Mus. Rey. No. 3495. 
Suborder TREPOSTOMATA, Ulzich. 
In nearly every respect this suborder is to be considered pre-eminent among the 
Bryozoa of the Lower Silurian rocks ; and this is as true of those in the lower horizons 
of the system as developed in Minnesota, as it is of those that have been so long. 
known in the Cincinnati rocks. As was stated in the introduction, the lower beds 
holding Bryozoa in Minnesota are really very near the apparent inception of the 
class, the oldest forms known being from the Chazy. This fact should not be forgot- 
ten when the characters of the species and genera seem unstable and troublesome to 
classify. ‘That is to be expected, because, near their point of origin all classes of ani- 
mals sufficiently studied may be shown to have been unsteady in their development, 
new features having been introduced or dropped with surprising rapidity and fre- 
quency. Some of these, at first wavering characters, later on, when the class became 
fully established, assumed fundamental importance. 
