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 Semicoscinium 

 and Fenestralia of the Fenestellidm. As shown, this ridge is the superficial extension 

 of a double median lamina constructed upon the same plan as in the Rhinididyonidce. 

 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 Cystodidyonidce, and in 

 Reticulipora, d'Orbigny, a Cretaceous genus of the Cydostomata. If we add to these 

 peculiarities the general resemblance of the zocecial tubes of Phylloporinorto those of 

 ordinary Trepostomata, 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 enuivalent beds. 



Mus. Beg. No. 3495. 



Suborder TREPOSTOMATA, Ulrich. 



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 suflaciently 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. 



