70 
Rizoceras (?) sp. 
Plate XIV, figures 4A, B 
Living chamber short and tumid, with circular cross-sections. The 
width of the chamber at its base is 18 mm.; 8 mm. farther up its width 
is 22 mm.; above this level the width diminishes until a point 15 mm. 
above the base is reached, beyond which the shell is not preserved. 
At mid-height the chamber is crossed by a relatively strong, transverse 
line of growth, which curves downward along the median line of the ventral 
side so as to form a very shallow angle with sides diverging at 160 degrees. 
This angle evidently locates the position of the hyponomic sinus at some 
earlier stage of growth of the conch. Directly beneath this downward 
curvature of the transverse growth line, a single segment of the siphuncle 
projects from beneath the base of the living chamber, a short distance from 
its ventral margin. This segment is poorly preserved, but appears to 
have been nummuloidal. Considering the size of the living chamber, 
the septum at its base is rather strongly concave. 
Locality and Horizon. From North arm, Great Slave lake; in the 
Silurian, associated with Pycnostylus guelphensis and Pycnostylus elegans. 
Collected by George S. Hume, Geological Survey, Canada. 
Remarks. Living chambers similar to the Great Slave Lake specimen 
here described occur in the Racine of southeastern Wisconsin and of the 
adjacent parts of Illinois. 
At an elevation of 13 to 16 mm. above the suture at the base of the 
living chamber of the Great Slave Lake specimen, along the median line 
of its ventral side, there appears to be a slight outward curvature of the 
shell. Should this be verified by future discoveries of better specimens, 
it would not agree with Rizoceroid structure; but, as far as can be deter- 
mined from the single specimen at hand, its general structure is Rizoceroid, 
rather than Gomphoceroid. In a Poterioceroid shell, the living chamber 
should be depressed dorso-ventrally, and should be more elongate and more 
contracted toward the aperture. 
Orthoceras sp. 
Plate XIV, figures 5A, B 
Conch orthoconic, depressed slightly dorso-ventrally. At the smaller 
end of the specimen the lateral diameter is 8 mm. and the dorso-ventral 
one is 7 mm.; at the larger end the lateral diameter is 10 mm. and the 
dorso-ventral is 9 mm. The entire length of the fragment is only 11 mm., 
and in this length there are seven camerse. 
The sutures of the septa curve slightly downward along the ventral 
and dorsal sides of the phragmacone, resulting in very shallow ventral 
and dorsal lobes and in very low ventral saddles. The curvature of the 
septa has a radius of about 10 mm. both laterally and dorso-ventrally, 
indicating that the dorso-ventral depression of the conch is not due to 
compression of the conch. The siphuncle is located along the dorso- 
ventral diameter, about one-sixth of the diameter of the shell in this 
direction from the ventral wall. It is narrowly cylindrical, and at the 
top of the specimen its diameter slightly exceeds half a millimetre. 
