110 THE PALEONTOLOGY OF MINNESOTA. 



[.Distribution. 



Isotrypa, Tedulipom, Semicoscinium, Ptiloporina, and Helicopora, have been described. 

 The Acanthocladiidie begin in the Lower Helderberg with a few species of Pinnatopora 

 and Idhyorachis, and Rhombopora, belonging to another family of the suborder, has 

 one species in the Niagara and several in the Lower Helderberg. The Cystodictyon- 

 idce, essentially a Devonian and Carboniferous family, is also met with for the first 

 time, a species of Dichotrypa having been described from the Niagara of Illinois, and 

 one or two of Cystodictya from the Lower Helderberg of New York. Rhinopora is 

 known only from the Clinton. 



The absence of so many characteristic Lower Silurian types, and the presence 

 of most of the genera that are strongly developed in the Devonian, proves, so far as 

 the Bryozoa are concerned, that the break between the Lower and Upper Silurian is 

 sharper than the one between the Upper Silurian and the Devonian. 



DEVONIAN SYSTEM : Several hundred species of Bryozoa have been described 

 from the rocks of this age. The great mass of these are Cryptostomata, and of these 

 the majority belong to the Fenestellidtr. Every genus of this family, excepting 

 Fenestralia, Lyropora and Archimedes, is more or less largely represented. To the 

 same suborder belong Cystodidya , Dichtyotrypa, Pi'ixwopora, Scalaripora, Coscinium 

 Tceniopora, Glyptopora, and Acrogenia, of the Gystodictyomdie ; a Corniferous species of 

 Ptilodidya (the last known of the genus), Taeniodidi/a, Intrapora, and Coscinella, of 

 thePtilodidyonida>;Euspillopora, ottheRhinididyonida'; Rhombopora, Nemataxis, Acan- 

 thoclema, and Badropora, of the Rhabdo/esontid j ; Streblotrypa ; and Pinnatopora and 

 Ptilopora of the Acanthocladiidw. The Cydostomt are included in the genera Clono- 

 pora, Cystopora, Hederella, Hernodia, and Reptaria; Ascodidyon represents the Cteno- 

 stomata. The Trepostomata are represented chiefly by numerous species of Fishdipont 

 and one or more of Eridopora, Chilotrypa, Meelcopora, Strotopora, Lichenotrypa, Busk- 

 opora, Selenopora, Pinnacotrypa, Botryllopora, Monotrypella, Amplexopora, Petalotrypa, 

 Batostomella, Leioclema, and Dekayia, 



SUB-CARBONIFEROUS SYSTEM: The Bryozoa of this age are very similar to those 

 of the Devonian, and the majority of the genera of either are common to both sys- 

 tems. The principal difference is found in the absence of some of the peculiarly 

 modified Devonian types of the Fenestellida', like Unitrypa, Loculipora, Fenestropom, etc. 

 They are, however, replaced by the equally interesting'genera Archimedes, Lyropora 

 and Fenestralia. Among the Cystodidi/onida; we miss Scalaripora and Acrogeniti, but 

 their vacant places are more than filled by the remarkable genera Evadinopora and Ac- 

 tinotrypa. Other Cryptostomata are Tceniodidya, ? Stidoporella, Pinnatopora, Septopora, 

 Ptilopora, Diplopora, Sphragiopora, Codoconus, Rhombopora, Badropora, Acanthoclema, 

 StreblotrypaECyclopora, Proutella and Heliotrypa. The three last named, together 



