SUB-CLASS I TETRABRANCHIATA 511 
called lateral lobes when occurring on the sides, and when on the venter or 
dorsum are termed ventral or dorsal lobes and saddles. The annular lobe is a 
small median dorsal lobe, usually pointed and occupying the centre of the 
main dorsal lobe. It is supposed to have had some relation to the correspond- 
ing inflection or point of the annular muscle among the Nautiloidea. In more 
specialised shells it is associated with a conical inflection of the septum itself. 
The curves are undulatory as a rule, but in some genera may be more or less 
angular. 
The position of the siphuncle does not enable one to\determine which is 
the ventral and which the dorsal side in most genera, but the hyponomic sinus 
in the aperture and the curved lines of growth are an almost unfailing index 
of the ventral side. The siphuncle is apt to change its position in the same 
individual at different stages of growth, but in shells of the same age it is 
approximately constant, and is available for diagnostic purposes in a number 
of genera. 
The siphuncle is variable in form and characteristics among Palaeozoic 
genera, being tubular in some (Fig. 1061), or inflated in the interseptal spaces 
in others, in such manner as to resemble a string of beads, or swollen discs 
which are separated by narrow constrictions (Fig. 1077). When of consider- 
able width, its cavity is partly filled up with thin calcareous lamellae (Fig. 
1088), partly with the calcareous cones immediately to be described (Fig. 
1056), or it is notably reduced by excretions around the interior of the 
funnels forming peculiar annular swellings known as rings, and which are 
generally composed of calcareous matter. The centre of the siphuncle in 
these forms is usually kept open more or less perfectly by an axial tube 
termed by Zittel the prosiphon (endosiphuncle of Hyatt), which will be con- 
sidered more fully in the descriptions of Hndoceras and <Actinoceras. In 
Diphragmoceras the siphuncle is septate like the shell. The upper parts of 
these large siphuncles were more or less unobstructed near the living chamber, 
and this part (the endoconal or siphuncular chamber of Hyatt) was doubtless 
occupied by an extension of the mantle cavity, probably containing portions 
of the viscera. 
The funnel of the siphuncle as described above is simple in structure, and 
is plainly directed towards the apex in all Nautiloids, with the exception of 
Nothoceras and its allies, the funnels (?) of which are turned in the opposite 
direction. The funnels, as a rule, are short and incomplete, although in 
the early stages of development of many shells, and in the adult stage of 
primitive forms they may be complete, extending from one septum to the next 
following (Fig. 1056), or even to the second preceding this (Fig. 1055, C). 
When the funnels are complete they are always contracted apically, and 
inserted one within the other. The siphuncle in most Nautiloids, as in the 
existing Nautilus (Fig. 1074), is apt to be more or less dilated in the younger 
stages, especially in the second and first air-chambers, and it is closed at the 
end within the first air-chamber by what is termed the caecum. The external 
shell is perforated by an elongated scar or cicatrix (Fig. 1073), closed by a 
plate, against some part of which the bottom of the caecum impinges in the 
interior. The presence of the cicatrix, as already stated, leads to the inference 
that a deciduous embryonal shell or protoconch must have been present. The 
shell on the apex is so much thinner than at later stages, and is so easily 
abraded or destroyed, and the cicatrix itself in consequence so slightly marked 
