BRYOZOA. 109 
Distribution ] 
of structure than is known from any of the more southern and eastern localities for 
the group. 
Among the Cryptostomata, both the Ptilodictyonidw and Rhinidictyonidew reach 
their maximum development in this group, while the Arthrostylide and Phylloporin- 
ide are both well represented. The T'vrepostomata likewise are strongly represented, 
and in the Minnesota shales of the group every family of the suborder has been rec- 
ognized, The Cyclostomata come in with Stomatopora proutana, a species that is con- 
tinuously present to the top of the Lower Silurian, and Berenicea. 
In the Trenton limestones and shales proper, the Cryptostomata have lost some 
of their strength, whilst that of the Trepostomata is increased by the addition of 
several genera——Prasopora, Monticulipora, Stellipora and Diamesopora. The Cyclo- 
stomata add Protocrisina, Diploclema, Scenellopora, and Diastoporina. 
Nearly all the genera now introduced continue to the top of the Lower Silurian, 
and before the close of the era we find a representative of the last of the five sub- 
orders, the Chilostomata, in a species of Pualeschara. The Trepostomata, however, 
again add greatly to their numbers in the Cincinnati group, in which nearly 200 
distinguishable forms of this suborder are known to me. These belong to 35 genera, 
giving every family, with the exception of the Fistuliporida, a strong representation. 
Of the Cyclostomata also the indivividuals and species became more numerous, while 
the Ctenostomata added another species of Vinella and the new genus Rhopalonaria. 
Upper Siturian System: The Bryozoa in the rocks of this system are very 
different from those of the Lower Silurian. The Tvepostomata are greatly reduced 
by the almost total extinction of the families Monticuliporide and Heterotrypide, and 
a considerable reduction in the Calloporide, Amplexoporidw, Diplotrypide, and the 
Ceramoporide. But the Fistuliporide, a family that reached its greatest development 
in Devonian and Subcarboniferous times, became prominent here. Of the Cyclosto- 
mata we have only Diploclema sparsum, a Niagara fossil, of the Ctenostomata, a few 
species, and of the Chilostomata, likewise only a few forms of Paleschara. But the 
Cryptostomata inaugurate a new and vigorous start. Two new genera, Clathropora 
and Stictotrypa, are added to the Ptilodictyonidw, while the genera Ptilodictya, and 
Phenopora, of the same family, became fully established. Rhinidictya, Pachydictya 
Phylloporina, Drymotrypa, Helopora and Nematopora, belonging to three other families 
of the suborder, are also well represented. The Fenestellidw, of which but a single 
Lower Silurian species is known, increase in abundance and variety from the Clinton 
to the Lower Helderberg, in which most of the generic types of the family, some of 
them, however, not yet fully established, are already distinguishable. Numerous 
species of Fenestella and Polypora, and one or more each of Unitrypa, Hemitrypu, 
