ARCTIC ORDOVICIAN AND SILURIAN CEPHALOPODS 281 
part of the dorsal side. Where the lateral diameter of the conch 
is 40 mm., the dorso-ventral diameter is 30 mm., the passage of 
the siphuncle through the septa scarcely equals 6 mm., and 
the distance of this siphuncle from the ventral wall is 2.5 mm. 
In form, the siphuncle is tubular, not enlarging between the 
septa. This is at variance with the structure of either Leuror- 
thoceras or of Orthoceras capitoUnum. Along the median part 
of the dorsal side the sutures curve distinctly upward. 
In Tripleuroceras rohsoni Whiteaves, from Stonewall, Mani- 
toba, and regarded by Whiteaves as Niagaran, the ventral side 
again is flattened, and the sutures of the septa curve downward 
here as in Leur orthoceras. Moreover, Whiteaves states that in 
the. Manitoba species the sutures are ‘^nearly straight where they 
pass over the sides. ’’ If the term straight here is equivalent to 
horizontal then the general' appearance of the sutures in this 
species should be similar to that in Leur orthoceras also dorsally. 
In regard to the siphuncle of the Manitoba species, however, 
Whiteaves states that ^Tt would have been better to say, that 
its exact shape, size and relative position are not at all clearly 
shown in the few specimens yet collected, though it seems to have 
been marginal and enlarged between the septa. Judging 
from one of the figures accompanying Whiteaves’s description 
of the Manitoba species, the siphuncle of the latter must have' 
been large and different in character from Leur orthoceras. 
In typical Tripleuroceras, founded by Hyatt on Orthoceras 
archiaci Barrande, from the Silurian of Bohemia, the cross- 
section of the conch is very similar to that of Leur orthoceras 
but the siphuncle is almost in contact with the ventral wall of 
the conch, its segments are distinctly nummuloidal, and the 
interior of the siphuncle is filled by vertical radiating plate- 
like deposits. Whether Tripleuroceras rohsoni has the same 
type of structure remains to be determined. 
In the Silurian genus Mixosiphonoceras, founded by Hyatt on 
Cyrtoceras desolatum Barrande, from the Silurian of Bohemia, 
the cross-section of the conch is triangular, but the siphuncle 
is situated not close to a flattened ventral side but near the 
unpaired angle of the triangle. Without a knowledge of the 
