CEPHALOrODA. 331 



occupation which is shut out by the horny cincture, and muscles of 

 adhesion from the rest of tliat cavity. 



Whatever additional advantage the existing Nautilus might derive, 

 by the continuation of a vascular organised membranous siphoij 

 through the air-chambers, in relation to the maintenance of vital 

 harmony between the soft and testaceous parts, such likewise must 

 have been enjoyed by the numerous extinct species of the tetra- 

 branchiate Cephalopods, which, like the Nautilus, were lodged in 

 chambered and siphoniferous shells. 



If the Nautilus extended itself in a straight line during its growth 

 instead of revolving round an imaginary axis, a straight conical shell 

 would be produced, with the chambered part divided by simjile septa 

 concave next the outlet. Such are, in fact, the characters of the 

 fossil shells called Orthoceratites. The siphon is usually central, but 

 sometimes marginal ; it is testaceous thi'oughout, and generally moni- 

 liform or dilated at each chamber ; this structure. Dr. Buckland re- 

 marks, would admit of the distension of a membranous siphon. Mr. 

 Stokes has discovered in a species of Orthoceratite from Lake Huron, 

 a second calcareous siphon included Avithin the moniliform siphon, 

 which shows no corresponding partial dilatations, but is connected 

 to the external siphon by successive series of radiating tubuli ; the 

 inner siphon was probably produced by a calcification of the mem- 

 branous siphon, and of processes continued from it at regular intervals. 

 The complex siphons of these OrthocercB are of great relative capacity, 

 and the species so characterised have been separated fi-om the simple 

 Orthoceratites under the subgeneric name Actmoceras. 



I have already alluded to the slightly undulating contour of the 

 margins of the septa in the Nautilus, which makes the surface next 

 the aperture of the shell convex at one part and concave at another. 

 In an extensive genus of extinct chambered shells called ' Am- 

 monites,' the sinuosity of the margins of the septa is much greater, 

 and most of the surface next the outlet is convex : the siphon per- 

 forates the septa at their centre in extremely few species, and in the 

 rest is situated at that margin which is next the outer curve or cir- 

 cumference of the shell. Certain chambered shells thus character- 

 ised are straight, like the Orthoceratites, but generally compressed, 

 with their numerous septa joining the outer shell by foliated denta- 

 tions : they are termed Baculites. In the true Ammonites the shell 

 is discoid and coiled upon itself as in the Nautilus ; but it is strength- 

 ened by arched ribs and dome-shaped elevations on the convex sur- 

 face, and by the tortuous windings of the foliated margin of the 

 transverse partitions. Separate casts of the interior of the chambers 

 are not unfrequently obtained, which have become detached by the 

 solution of the calcareous walls and septa of the shell, or are held to- 



