Dec, 1894, CEPHALOPOD BEGINNINGS. 423 



regular septa was a gradual development from a primitive non-septate 

 condition. 



A peculiarity that distinguishes the septate cephalopod shell 

 from the septate shells or tubes of most of the other animals above 

 alluded to, is the fact that an extension from the soft body always 

 passes backwards through the septa and, however long the phragma- 

 cone may be, always appears to maintain connection with its apex. 

 In such a form as the modern Nautilus, this extension comes off 

 sharply from the visceral hump in the form of a thin tube, which 

 passes immediately through the centre of the last-formed septum and 

 stretches back through the loculi and through the centres of all the 

 preceding septa. This fleshy tube is known as the ' siphuncle.' It 

 will, however, be noticed that, just before the siphuncle becomes 

 attached to the visceral hump, that is to say just where it passes 

 through the last septum, it enlarges slightly, so that between the 

 visceral hump and the siphuncle proper there is a small cone, which 

 contains a slight extension of the generative viscera, and which may 

 therefore be distinguished as the * visceral cone.' Were all cephalo- 

 pods fashioned like Nautilus in this respect, there would be no need 

 to use this term ' visceral cone ' ; but there are many in which the 

 visceral cone assumes very large proportions and appears to have 

 given rise to some misunderstanding in the minds of those who have 

 described such cephalopods. If we define the visceral cone as that 

 part of the soft body which lies posteriorly to the inner margin of the 

 last-formed septum, but anteriorly to the narrow siphuncle, we shall 

 at once render futile many discussions that have taken place, as, for 

 instance, those concerning the presence or absence of a siphuncle in 

 Sepia, and we shall have a ready explanation for many puzzling 

 structures in archaic cephalopods, such as the ' sheaths ' and absurdly 

 named ' endosiphon ' of Pilocevas and Endocevas. These, however, 

 are details rather foreign to the present purpose. It is enough to 

 point out that, whereas the siphuncle proper does not secrete a 

 definite shell and merely becomes impregnated to a slight extent with 

 carbonate of lime, the visceral cone, on the contrary, retains to a very 

 large extent, in many cases, the typical secretive power of the visceral 

 hump, by which shell-substance is eventually formed. 



Having considered the phragmacone, or conical-chambered 

 conch, in sufficient detail for the object of this paper, we turn now 

 to a structure to which, under Owen's name of ' protoconch,' much 

 attention has of late years been directed. Examination of the 

 phragmacone of Spivula, of Belemnites, or of any ammonite or, better, 

 of such a loosely coiled goniatite as Mimoceyas coiiipyessum, will show 

 that at the apex of the cone, which of course lies in the centre when 

 the shell is coiled, there is a little round bulb, usually separated from 

 the conch by a sliglit constriction (compare Figg. i, a and 2, h). The 

 siphuncle passes through the end of the conch into the protoconch ; 

 but instead of retaining its simple tubular character, it generally 



