ARCTIC ORDOVICIAN AND SILURIAN CEPHALOPODS 
289 
18. Actinoceras amundseni Sp. nov. 
Plate XXIX, figs, lA, IB; Plate XXXII, fig, 3 
Specimen exposing the ventral side of the cast of the interior 
of the phragmacone, including the sutures of the septa, and 
exhibiting also the dorsal side of the siphuncle. 
The diameter of the conch, as far as can be determined from 
the transverse curvature of its ventral side, is estimated 
at 70 mm. Apparently there is a slight tendency toward angu- 
larity along the median part of this ventral side. It is estimated 
that 6 camerae occupy a length equal to the diameter of the 
conch. The sutures curve strongly downward on its ventral 
side, forming an angular ventral lobe or hyponomic sinus with 
sides diverging at an angle of 140 degrees. The septa are deeply 
concave. 
The siphuncle is almost in contact with the ventral wall of 
the conch, being 1 or 2 mm. distant from the latter. The maxi- 
mum diameter of the segments of the siphuncle is 28 mm., 
narrowing to 19 mm. at the grooves separating the segments. 
Where the height of the segments is 13 mm., the septa are adnate 
to the lower part of these segments for a height of 5 mm. ; along 
this part of the segments the vertical outline of the latter is nearly 
straight or only faintly concave. Above the point of departure 
of the septa, the vertical outline of the segments is strongly 
convex, reaching its maximum 7 or 8 mm. above the base of the 
segments. The upper surface of the segments forms an angle 
of 105 or 110 degrees with the vertical axis of the conch, rising 
toward its ventral side. No trace of surface markings remains. 
Locality and Horizon. — From some unknown locality, pro- 
bably of Black River age, either on Boothia Felix or on Prince 
William Land. In the Palaeontologisk Museum, Kristiania. 
Collected in 1903-04 by Lieut. Godfred Hansen, on the expedi- 
tion of the Gjoa, led by Captain Roald Amundsen, in whose 
honor this species is named. 
Remarks. — In Actinoceras amundseni slightly more than 2 
segments of the siphuncle occur in a length equal to the maxi- 
mum diameter of these segments. This is true also of Huronia 
