71 
Locality and Horizon. From the southern part of the North arm, 
Great Slave lake, in a cherty fragment of rock of Ordovician age. Collected 
by George S. Hume, Geological Survey, Canada. 
Remarks. Distinctly depressed orthocones, with shallow ventral 
and dorsal lobes, are known from various horizons and localities. Until 
their internal structure has been studied it is not safe to assume that they 
are cogeneric. 
In Orthoceras ekwanense Whiteaves, from the Silurian, Portage road, 
Ekwan river, west of James bay, a similar disposition of saddles and lobes 
is noted. 
The Great Slave Lake specimen here described is distinguished by the 
elose proximity of the siphuncle to the ventral wall of the conch; apparently 
this siphuncle was of small size, judging from the size of its passage through 
the septa. 
Trocholites sp. 
Plate XIY, figures 6A, B, C 
Specimen consisting of three-fourths of a volution of some strongly 
curved conch, apparently nautiloid in structure. An open space, 4 • 5 mm. 
in diameter, partly enclosed by that part of the volution which remains, 
-evidently was occupied originally by the apical part of the conch. To 
what extent this central space was filled is not known, but the cross-section 
of the fragment is circular, and there is no evidence of an impressed zone 
along the dorsal margin of the whorl. Apparently the rate of curvature 
of this fragment of the conch is such that later growths could not have 
continued without interference by the more apicad parts of the shell. If 
such interference actually took place the later formed whorls may have 
had an impressed zone as in Trocholites, but no evidence of such a structure 
is at hand. The chief reason for assigning this fragment to the Trocholitidae 
Tather than to the Tarphyceratidae is on account of its circular cross- 
section, there being no evidence of lateral compression as in the latter 
family. 
The specimen consists of three parts. At the smaller end, a cast of 
the interior exposes a little over three camerse. Adjoining this is an equal 
length in which the phragmacone retains the surface features of the shell. 
The remainder consists of a cast of the exterior of the specimen, also 
retaining the surface ornamentation of the latter. All of the cross-sections 
are circular. 
The septa are distinctly concave in a dorso-ventral direction, but are 
only slightly concave in a lateral direction. The radius of curvature 
in a dorso-ventral direction is estimated at 3 mm., that in a lateral direction 
is about 15 mm. This results in distinct dorsal and ventral saddles and 
lateral lobes. The siphuncle is located slightly less than one-fourth of 
the dorso-ventral diameter of the phragmacone from the ventral wall. 
It is relatively small and tubular. Where the dorso-ventral diameter of 
the phragmacone is 3 mm., the diameter of the siphuncle appears to be 
scarcely one-fifth of a millimetre. About eight camerse occupy a length 
of 7 mm., at the apical end of the species. 
