65 
CEPHALOPODA FROM THE ORDOVICIAN AND SILURIAN OF 
GREAT SLAVE LAKE 
By A. F. Foerste 
CONTENTS 
Page 
Introduction 65 
Description of species 66 
Illustration 
Plate XIV. Illustrations of fossils 127 
INTRODUCTION 
Among the specimens collected by George S. Hume, Geological 
Survey, Canada, along the west shore of the North arm, Great Slave lake, 
are several fragments of cephalopods which are not sufficiently well pre- 
served to serve as types of new species; nevertheless they are of sufficient 
interest to merit description and figuring. The character of the associated 
fauna is indicated in the accompanying report by Mr. Hume. 
The three specimens here referred to Endoceras, Orthoceras, and 
Trocholites , Plate XIV, figures 1, 5, and 6, were found along the southern 
part of the northern arm of Great Slave lake. Of these three specimens, 
the Endoceras indicates an Ordovician horizon. The other two specimens 
referred to Orthoceras and Trocholites, occur in cherty rock from the same 
locality, and although neither of these two specimens, in our present 
knowledge of American cephalopods, can be utilized in determining the 
age of the containing rock, the latter also is regarded as of Ordovician 
age. 
Plate XIV, figures 2, 3, 4, show the remaining three specimens, here 
referred to Phragmoceras, Poterioceras, and Rizoceras . They are found 
associated with Pycnostylus guelphensis and Pycnostylus elegans . These 
two corals originally were described by Whiteaves from the Guelph of 
southern Ontario, and both were identified by Lambe from the Silurian 
of the portage road at the fall on Ekwan river, west of James bay, in northern 
Ontario. Pycnostylus guelphensis is listed by Whiteaves from the Silurian 
also, at Davis point, lake Manitoba; it occurs too in the Racine of south- 
eastern Wisconsin and of the adjacent part of Illinois. The specimens 
here referred to Poterioceras and Rizoceras are closely related to undescribed 
species of cephalopods occurring in the Racine. The specimen referred 
to Phragmoceras appears to belong to an erect species, little curved length- 
wise, different from any species so far discovered in American strata. 
The general horizon for this part of the Silurian on Great Slave lake 
appears to be Guelph or Racine. 
