2 BRYOZOAN FAUNA OF THE ROCHESTER SHALE. 
Unless otherwise designated, the photographs illustrating this paper were made in the 
laboratory of the United States National Museum and the drawings are the work of the 
writer. 
From the amount of material studied, the bryozoan fauna of these shales is believed to 
be represented here with some degree of completeness. 
GENERAL DISTRIBUTION OF SILURIAN BRYOZOA. 
The Silurian bryozoa as a whole have a facies quite distinct from the preceding and suc- 
ceeding systems. When they are compared with the Ordovician types, the noticeable 
features are the predominance of the Cryptostomata and the decline of the Trepostomata. 
The Cyclostomata continue to hold their own, while the Ctcnostomata and Chilostomata, 
as in all the other Paleozoic systems, have few representatives. In the Devonian the 
Cryptostomata still predominate, the Cyclostomata have been greatly augmented by species 
of the Fistuliporidae, and the Trepostomata, still further reduced in number of species, are 
represented mainly by genera of the long-enduring Batostomellida?. 
Conditions during Oswegan times were in general so unfavorable for the preservation 
of fossils that it is not surprising that few bryozoa are known from these rocks. The same 
conditions, with a slight modification, existed in the Medina, where, however, a small 
bryozoan fauna is known but is as yet unpublished. This consists of a few species of 
Trepostomata and Cryptostomata which have been found in the more calcareous strata 
of the formation. 
The Clinton and Rochester divisions of the Niagaran group afford species and specimens 
of this class in great abundance, but in the succeeding Lockport and Guelph divisions the 
bryozoa are comparatively rare. In New York and Ontario the thin limestone layers of 
the Clinton are often crowded with Helopora fragilis and several species of Phsmopora, 
these being the characteristic bryozoa in this region. In Ohio the Clinton beds contain a 
considerable number of species, but the predominating forms are Rhinopora verrucosa, 
species of Pachydictya and Phsenopora, and a few large ramose Trepostomata, some of 
which seem to be wanting in the East. The unusual development of the genus Phsmopora 
is the striking feature of Clinton time. During the deposition of the Rochester shale, ideal 
conditions for the development of bryozoa seem to have been reached. Near the middle 
part of this formation occurs a stratum several feet in thickness that is literally one mass 
of fragments of these organisms. The correlation of these shales with the Osgood beds 
in the Indiana Niagaran section and their faunal similarity to the Buildwas beds of the 
Wenlock of England is discussed later in this paper. As already stated, bryozoa are 
apparently rarely found in the Lockport and Guelph divisions of the Niagaran. 
The Salina and Rondout (Bertie) formations of the Cayugan group were deposited under 
conditions unsuited to bryozoan life. During the time of the last formation of this group, 
the Manlius, conditions more favorable to the life of the class returned, a fauna of fifteen or 
twenty species being known. This will be described by Mr. E. O. Ulrich and the writer in 
a forthcoming volume of the Maryland Geological Survey. 
The various formations of the Helderberg again, especially the New Scotland, are rich 
in bryozoa. Most of these have been described and illustrated by Hall and Simpson in 
Volume VI of the Paleontology of New York. Although this series of rocks is referred by 
some authors to the Devonian, the similarity of its bryozoan fauna to that of the Niagaran 
is so great that the writer can not see that its position should be other than in the Silurian. 
GENERAL DISCUSSION OF THE ROCHESTER SHALE AND ITS 
BRYOZOAN FAUNA. 
Succeeding the Clinton in western New York and Ontario is a shale, which on account of 
its prominence along Genesee River at Rochester, N. Y., was termed the Rochestep shale. 
In the Niagara region this shale has a thickness of about 70 feet and is divisible into a lower 
and an upper half. The lower half is generally highly fossiliferous and contains numerous 
