MINERALS AND GEOLOGY OF CANADA. 23 
latter species (after Billings), from the Black River Limestone of La 
Cloche Island, Lake Huron. 
Fig. 133. Fig. 134. 
a. Represents the aperture. 
To this genus, Hall’s Oncoceras constrictum (fig. 134) should also 
be referred. This species is exceedingly common in the low part of 
the Trenton group; but when in imperfectly preserved specimens, it 
cannot be distinguished from a cyrtoceras. The siphuncle is near the 
outside or larger curve of the shell. 
Iituites:—The shell in this genus, is involute or “rolled v 
(like that of the ‘nautilus) for a certain distance, and is then pro- 
jected in a straight line. The septa are simply concave, and the 
siphuncle of small size and mostly central. Species have not been 
found as yet above the Silurian rocks. Jn fragmentary specimens, 
however, it is often impossible to deter- 
mine the genus—the straight part of the 
shell resembling that of an orthoceras 
with narrow siphuncle, and the involute 
portion being identical with the shell 
of a nautilus. Fig. 135 represents the 
Intuites undatus of Hall. Examples 
having a general resemblance to this, 
but (as first pointed out by the writer) 
with external siphuncle, occur in our 
Lower Silurian beds, at Lorette near Quebec, and elsewhere. 
Nautilus.—This genus is one of peculiar paleeontological interest, 
as the only living type of the great group of tetrabranchiate cephalo- 
pods, or those inhabiting a many-chambered shell. It passes (although 
with diminished, and, of course, with changing species) from the Silu- 
rian epoch into the existing age—its fossil representatives traversing 
