30 A POPULAR EXPOSITION OF THE 
Orthoceras.—In this genus the shell is straight and conical, taper- 
ing more or less gradually from the body chamber to its other 
extremity. The septa are simply concave, or slightly sinuous, and 
at comparatively short distances apart. If we imagine the shell of 
a nautilus unrolled and straightened out, we have the typical ortho- 
ceras shell. The siphuncle is variable, both in shape and position. 
Three convenient, if not strictly natural, sub-genera, Orthoceras 
proper, Ormoceras, and Hndoceras, may be founded on its characters. 
The genus ranges from the Lower Silurian into the Triassic forma- 
tion. In many of its examples, the shell, if perfect, would shew a 
length of several feet. 
The first sub-genus, Orthoceras proper, has a siphuncle in the form 
of a narrow tube, central or sub-central in position. O. lammellosum 
(fig. 127) and O. bilineatum (fig. 128), both from Lower Silurian 
Strata, are Canadian examples of common occurrence. 
The second sub-genus, Ormoce- 
yas, comprises the various ortho- 
ceratites (as the species of the 
genus Orthoceras are collectively 
termed) with moniliform or ‘“ bead- 
ed” siphuncle, as shewn in Or- 
moceras tenuifilum (fig. 129) from 
the Trenton hmestone and lower 
beds. This sub-genus includes the 
Huronia and Actinoceras of au- 
thors, and also the peculiar flat- Me ete Fig: 128; 
tened species named Gonioceras anceps by Hall. This latter form is 
an Orthoceras with beaded siphuncle and slightly sinuous septa, and 
with a shell so compressed as 
to offer almost trenchant edges. 
Fig. 130 represents a fragmen- 
tary specimen. The species is 
very common in the Chazy and 
Black river limestones at the 
lower part of the Trenton group. 
In weathered specimens, both of 
this and other species of Ormo- 
Fig. 199. ceras, the outer portion of the Fig. 130. 
A worn fragment shell is often obliterated, when the beaded siphuncle 
shewing siphuncle , A i 
and septa. with its attached septa, has a certain resemblance to 
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