ARCTIC ORDOVICIAN AND SILURIAN CEPHALOPODS 299 
The surface of the conch apparently was smooth. 
Locality and Horizon. — From the Lower Rapids on the Sha- 
mattawa River, in northern Manitoba. In the Ordovician. 
No. 308S, in the Royal Ontario Museum of Paleontology at 
Toronto. 
Remarks. — This species is characterized by the relatively long 
tubular septal necks separating the relatively narrow, abruptly 
projecting annulations. 
25. Cyclendoceras annulatum (Hall) 
Plate XXXI, fig, 8 
Endoceras annulatum Hall, Pal. New York, 1, 1847, p. 207, 
pi. 44, figs, la, lb. Cyclendoceras annulatum Grabau and 
Shimer, N. Am. Index Fossils, 2, 1910, p. 43, fig. 1241. 
Type specimen. — Phragmacone 225 mm. in length, retaining 
26 camerae in a length of 22 mm.; of these the lower 5 occupy 
at length of 38 mm. and the upper 5 a length of 42 mm. Since 
only 16 transverse annulations occur in this length of 26 camerae 
it is evident that the rhythmic enlargement of the aperture of 
the shell at the annulations has no connection with the periodic 
withdrawal of the lower part of the animal from contact with 
the basal part of the living chamber. The specimen enlarges 
from a diameter of 59 mm. at its base to 70 mm. at a point 185 
mm. farther up, or at the rate of 6 mm. in a length of 100 mm., 
thus indicating an apical angle of 3.5 degrees. About 7 camerae 
occur in a length equal to the diameter of the conch. The 
cross-section of the conch is nearly circular. 
The sutures of the septa are directly transverse or only slightly 
lower on the ventral side. The concavity of the septa is about 
one-fifth of the diameter of the conch. On the ventral side of the 
conch the vertical sections of the septa have straight courses from 
the wall of the conch to within 3 mm. of the siphuncle. On the 
dorsal side the concave curvature of the septa, from the wall of the 
conch, to within 3 mm. of the siphuncle, only slightly exceeds 
1 mm. Beginning 3 mm. from the siphuncle, the inner parts 
of the septa curve strongly downward, coming in contact with 
