r, , „ .• •, 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 Ptilodictyonidce and RUnididyonidce reach 

 their maximum development in this group, while the Arthrostylidce and Phylloporin- 

 idce are both well represented. The Trepostomata 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 Gyclo^ 

 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 Paleschara. The Trepostomata, however, 

 again add greatly to their numbers in the Cincinnati group, in which nearly 200 

 distinguishable forme of this suborder are known to me. These belong to 35 genera, 

 giving every family, with the exception of the Fistuliporidce, 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 Silurian System : The Bryozoa in the rocks of this system are very 

 different from those of the Lower Silurian. The Trepostomata are greatly reduced 

 by the almost total extinction of the families MonticuUporidce and Heterotrypidce, and 

 a considerable reduction in the Calloporidce, Amplexoporidce, Diplotrypidce, and the 

 Ceramoporidce. But the Fistuliporidce, 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 Ptilodictyonidoi, while the genera Ptilodictya, and 

 Phcenopora, of the same family, became fully established. Bhinidictya, Pachydictya 

 Phylloporina, Drymotrypa, Helopora and Nematopora, belonging to three other families 

 of the suborder, are also well represented. The Fenestellidce, 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, Hemitrypa, 



