592 MOLLUSCA phylum vi 



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 diff'erent stages of development, but in shells of the same age 

 it is approximately constant, and is available for diagnostic purposes in a 

 number of genera. 



The siphuncle differs in its form and characteristics among Paleozoic 

 genera, being tubulär in some (Fig. 1110), 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. 1126). When of consider- 

 able width, its cavity is partly filled up with thin calcareous lamellae (Fig. 

 1137), partly with the calcareous cones immediately to be described (Fig. 

 1105), 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 (endosiphunde of Hyatt), which will be con- 

 sidered more fully in the descriptions of Endoceras and Adinoceras. 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 diamher 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. 1105), or even to the second preceding this (Fig. 1104, 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. 1123), 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. 1122), closed by a 

 plate, against some part of which the bottom of the caecum iropinges 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 

 even in perfect shells, that good examples are rarely found, and when met 

 with require careful preparation and close Observation. 



