ARCTIC ORDOVICIAN AND SILURIAN CEPHALOPODS 
295 
22. Actinoceras cf. tenuifilum centrale 
Plate XXVIII, fig. 4; Plate XXXII, fig. 5 
Specimens cut vertically through the center of the siphuncle, 
and polished to show the structure of the interior of the conch. 
The opposite part of the specimen ends abruptly within the 
matrix along a vertical plane passing along the most distant 
margin of the siphuncle in a direction parallel to the polished 
section, as though part of the specimen had weathered away 
before fossilization. The apical angle of the conch is estimated 
at 7 degrees. At the base of the specimen the width of the conch 
is 48 mm.; 8 camerae occupy a length equal to the diameter of 
the conch at the top of the series being counted. The septa are 
moderately concave, the amount of concavity equalling 10 mm. 
in a width of 48 mm. at the base of the specimen. At this point 
the maximum diameter of the segments of the siphuncle equals 
23 mm., diminishing to 17.5 mm. at thelowei: margin of the septal 
necks. The siphuncle is moderately excentric in position; as 
far as may be determined from the specimen at hand, its dis- 
tance from the ventral wall at the base of the specimen is about 
9 mm., compared with a distance of 15 mm. from the dorsal 
wall. The septal necks descend about 1 mm. below the general 
curvature of the septa, the intervals between successive seg- 
ments of the siphuncle equalling about 1 . 5 mm. 
Locality and Horizon. — From some unknown locality, prob- 
ably of Black River age, either on Boothia Felix or on King 
William Land. Collected in 1903-04 by Lieut. Godfred Hansen. 
In the Palaeontologisk Museum, Kristiania, Norway. 
Remarks. — Compared with either Actinoceras tenuifilum cen- 
trale or the variety ursinum the siphuncle of the specimen here 
described is relatively smaller. It is impossible to determine 
whether the siphuncle actually becomes narrower toward the 
top, as in Par actinoceras, or whether the appearance of narrowing 
in this direction is due to the section having been cut more or 
less oblique to the vertical axis. In the relative number of 
camerae this specimen approaches the variety ursinum. It 
probably is a distinct variety, but the specimen is inadequate 
for more exact discrimination. 
