110 THE PALEONTOLOGY OF MINNESOTA. 
(Distribution. 
Isotrypa, Tectulipora, Semicoscinium, Ptiloporina, and Helicopora, have been described. 
The Acanthocladiide begin in the Lower Helderberg with a few species of Pinnatopora 
and Icthyorachis, and Rhombopora, belonging to another family of the suborder, has 
one species in the Niagara and several in the Lower Helderberg. The Cystodictyon- 
ida, 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 Ienestellidw. very genus of this family, excepting 
Fenestralia, Lyropora and Archimedes, is more or less largely represented. ‘To the 
same suborder belong Cystodictya, Dichtyotrypa, Prismopora, Scalaripora, Coscinium 
Twniopora, Glyptopora, and Aerogenia, of the Cystodictyonidw; a Corniferous species of 
Ptilodictya (the last known of the genus), Tweniodictya, Intrapora, and Coscinella, of 
the Ptilodictyonide ; Euspillopora, of the Rhinidictyonida; Rhombopora, Nemataxis, Acan- 
thoclema, and Bactropora, of the Rhabdomesontidw; Streblotrypa; and Pinnatopora and 
Ptilopora of the Acanthocladiide. The Cyclostomata are included in the genera Clono- 
pora, Cystopora, Hederella, Hernodia, and Reptaria; Ascodictyon represents the Cteno- 
stomata. The Trepostomata are represented chiefly by numerous species of Fistulipora 
and one or more of Hridopora, Chilotrypa, Meekopora, 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 Menestellid, like Unitrypa, Loculipora, Fenestropora, ete. 
They are, however, replaced by the equally interesting genera Archimedes, Lyropora 
and Fenestralia. Among the Cystodictyonidw we miss Scalaripora and Acrogenia, but 
their vacant places are more than filled by the remarkable genera Hvactinopora and Ac- 
tinotrypa. Other Cryptostomata are Teniodictya, ? Stictoporella, Pinnatopora, Septopora, 
Ptilopora, Diplopora, Sphragiopora, Coeloconus, Rhombopora, Bactropora, Acanthoclema, 
Streblotrypa, Cyclopora, Proutella and Heliotrypa. The three last named, together 
