HKYO//JA. 



Distribution ] 



of structure than is known from any of the more southern and eastern localities for 

 the group. 



Among the Cryptostomata, both the Ptilodictyonidtr and Ehinididyonida> reach 

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

 idia 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 Cyclostornatd come in with Stomatopora proutana, a species that is con- 

 tinuously present to the top of the Lower Silurian, and Berenlcea. 



In the Trenton limestones and shales proper, the Cryptostotta have lost some 

 of their strength, whilst that of the Trepostomabt is increased by the addition of 

 several genera Prasopora, Monticuliporn, Stellipora and Diainesopora. The Cydo- 

 stoinatx add Protocrisina, Diploclemu, Scenellopora, and Diastoporimr. 



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 Chilostomatu, in a species of Paleschara. 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 Fistuliporidir, a strong representation. 

 Of the Cyclostomata also the indivividuals and species became more numerous, while 

 the Ctenostomaia added another species of Vinelln 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 Trepostom<d are greatly reduced 

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

 a considerable reduction in the Calloporidce, Amplexoporidw, Diplotrypidcc, and the 

 Ceramoporida'. Bub the Fistuliporidce, a family that reached its greatest development 

 in Devonian and Subcarboniferous times, became prominent here. Of the Cyclosto- 

 mutii we have only Diploclema sparsutn, a Niagara fossil, of the Ctenostomatii, 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 Ptilodictyonidce, while the genera Ptilodictya, and 

 r/ifi-noponi, of the same family, became fully established. Rhinidictyn, Pachydicfyu 

 I'tti/Iloporina, Drymotrypu, lieloponi and Neniatopora, belonging to three other families 

 of the suborder, are also well represented. The Fenestellidii', 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, Hemitrypo, 





