ARCTIC ORDOVICIAN AND SILURIAN CEPHALOPODS 301 
Annulated Endoceratidae, such as here are included in Cyclendo- 
ceras, are very well known in northern faunas. At least a dozen 
species have been studied. They occur in loose Black River 
blocks west of Cape Chidley at the northern end of Labrador, 
on the Nelson and Shamattawa rivers west of Hudson Bay, 
in Manitoba, and along that Arctic invasion which extended 
southwestward to the Black Hills of South Dakota, the Big 
Horn Mountains of Wyoming, the Teton range of Idaho, and 
the Canyon City area of Colorado. At least 6 species are known 
from this southwestern extension of the Arctic invasion. Two 
species occur in Wisconsin, and 2 occur in the Watertown area 
of New York, one of which, Cyclendoceras annulatum, a Trenton 
form', is the genotype. 
26. Cyclendoceras or Dawsonoceras sp. 
Plate XXXI, fig. 2 
Endoceras (Cyclendoceras) annulatum Hall, On Some Ordovi- 
cian Fossils from Boothia Felix and King William Land, collected 
during the Expedition of the Gjoa. Videnskapssilkapets Skriften, 
1912. PI. IV, hg. 1. 
A. Specimen figured hy Holtedahl. — Diameter at lowest well 
preserved annulation, 57 mm. Rate of enlargement of conch, 
3 degrees or less. Compressed, partly subsequent to death, 
the shorter diameter equalling about 77 per cent of the longer 
one. Five camerae occur in a length equal to the longer diame- 
ter of the conch, each camera being occupied by a transverse 
annulation whose crest is located slightly above mid-height of 
the camera. The sutures rise gently toward a point which is 
slightly toward the left of the median line of the published 
figure. Toward the right of this point, both sutures and annula- 
tions are almost directly transverse. Toward the left, both 
sutures and ^nnulations curve downward, but much more moder- 
ately than in typical Cyclendoceras. 
B. Specimen figured in this Bulletin {Plate XXXI, fig. 2) . — Seven 
annulations occur in a length equal to the diameter of the conch. 
These, annulations rise 1.5 mm. above the intervening grooves. 
