ARCTIC ORDOVICIAN AND SILURIAN CEPHALOPODS 
293 
of the specimen, where the diameter of the conch is 23 mm., 
to 4 at its top where the diameter of the latter is 33 mm. The 
diameter of the siphuncle at these points is 15 and 21 mm. 
respectively. The siphuncle either is in actual contact with the 
ventral wall of the conch or is scarcely more than 1 mm. distant- 
from the latter. The inner part of the groove separating suc- 
cessive nummuloidal segments of the siphuncle is so narrow and 
angular that the septal necks must have been short, possibly 
less than 1 mm. The septa are in contact with the lower side 
of the segments of the siphuncle. The nummuloidal segments 
incline at an angle of about 75 degrees with the vertical axis of 
the conch. 
Locality and Horizon. — From some unknown locality, prob- 
ably of Black River age, either on Boothia Felix or on King 
William Land. In the Palaeontologisk Museum, Kristiania, 
Norway. Collected by Lieut. Godfred Hansen in 1903-04. 
Remarks. — This species is characterized by its relatively 
large siphuncle and the relatively small number of camerae. 
In both of these respects, especially in the height of the camerae. 
it differs from Actinoceras higshyi Bronn, and it probably is a 
new species. 
The specimen figured by Holtedahl from the Boothia Felix — 
King William I^and area as Actinoceras beloitense Whitfield 
(Holtedahl, pi. IV, fig. 2) differs from all other forms here figured 
in its small siphuncle traversing relatively tall camerae. The 
number of annulations of the siphuncle in a length equal to its 
diameter is only two apparently. 
21. Actinoceras tenuifilum centrale Var. nov. 
Plate XXVIII, fig. 1; Plate XXXII, fig. 4 
Apical angle about 10 degrees; cross-section circular. Seven 
and a half camerae occupy a length equal to the diameter of the 
conch at the top of the series being counted. The sutures are 
almost directly transverse. The siphuncle is practically central 
in position, its distance fron the ventral wall being 2 mm. less 
than its distance from the dorsal wall of the conch. Its diameter 
