69 
So little of the surface of the exterior of the phragmacone is preserved 
that very little definite information is added to our knowledge of the conch. 
However, this little suggests that the conch was curved only slightly 
lengthwise, in a dorso-ventral plane, and its general aspect probably was 
erect. 
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. The relatively erect position and slight curvature of the 
conch, if verified by future discoveries of better specimens, will readily 
discriminate this Great Slave Lake species from all others hitherto 
described from American strata. For determining its relationship to 
other species a much better knowledge of the margin of the aperture is 
needed. 
Poterioceras (?) sp. 
Plate XIY, figures 3A, B 
Living chamber depressed dorso-ventrally; at its base the lateral 
diameter is 18 mm., and the dorso-ventral diameter is about 15 mm.; 
a short distance below the top of this chamber its width has increased 
to 21 mm., and its dorso-ventral diameter to about 19 mm. The height of 
the chamber, along its ventral side, is estimated at 20 mm. The living 
chamber is curved lengthwise, the radius of curvature on the convex, 
ventral side being 30 mm., and that on the concave, dorsal side equalling 
about 25 mm. 
The suture of the septum at the base of the living chamber is straight. 
The septum at this point is rather strongly concave, especially in a dorso- 
ventral direction. The siphuncle is situated close to the ventral wall, 
its centre being about 1 • 5 mm. from the latter. The form of the segments 
of the siphuncle can not be determined definitely, but it appears to have 
been more or less nummuioidal rather than tubular. 
The margin of the aperture is not distinctly defined. Along the 
median line of its ventral side, toward the margin of the aperture, the 
cast of the interior of the living chamber is strongly curved inward, suggest- 
ing an inward projecting dentate growth, a millimetre in length. It is 
not known whether this growth is characteristic of the species or represents 
only an aberrant feature confined to a single specimen. 
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. This Great Slave Lake species is a Poterioceroid only in 
the sense that it is a conch strongly depressed in a dorso-ventral direction. 
There is no evidence of strong contraction of the living chamber toward the 
aperture. It belongs to a group of species, forming a distinct undescribed 
genus, well represented in the Racine dolomite of southeastern Wisconsin 
and adjacent Illinois, and also in the Hopkinton dolomite of northwestern 
Illinois and adjacent Iowa. 
