CEPHALOPODA. ,595 



which the soft parts expose to the water, according as they may be 

 expanded to the utmost and spread abroad beyond the aperture of 

 the shell, or be contracted into a dense mass within its cavity. Tlie 

 Nautilus would likewise possess the additional advantage of pro- 

 ducing a slight vacuum in the posterior parts of the chamber of 

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

 adhesion from the rest of that cavity. 



Whatever additional advantage the existing Nautilus might derive, 

 by the continuation of a vascular organised membranous siphon 

 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- 

 brancliiate 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 simple septa 

 concave next the outlet. Such are the more obvious characters of 

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

 but sometimes marginal ; it is testaceous throughout, and sometimes 

 moniliform or dilated at each chamber ; this structure. Dr. Buckland 

 remarks, Avould admit of the distension of a membraneous siphon, f 

 Mr. Stokes J has discovered in a species of Orthoceratite (^Actino- 

 ceras) from Lake Huron, a second slender calcareous tube included 

 Avithin the moniliform siphon, which shows no corresponding partial 

 dilatations, but is connected to the external siphon by successive 

 series of radiating plates ; the inner siphon was probably produced 

 by a calcification of the membranous siphon, and of processes con- 

 tinued from it at regular intervals. The complex siphons of these 

 OrthocercB are of great relative capacity. In Ormoceras the siphun- 

 cular beads are constricted in the middle. In Gomphoceras the 

 shell is fusiform or globular, tapering towards the aperture of the 

 last chamber, which, is much contracted, so as to suggest that the 

 soft parts could not have been wholly i*etractible into that chamber. 

 The chevron-shaped coloured bands preserved on Orthoceras mi- 

 gtdifertim show that the shell, like that of Nautihis, was external. 

 The Orthoceratites are abundantly and widely diffused through the 

 paltcozoic rocks : specimens of the Orthoceras giganteum, of the 

 length of six feet, have been discovered in the carboniferous lime- 



* LXXXIV. p. 331. (1843). According to Vrolik (CCCXCIII.),the gas with 

 which the chambers are fiUed consists chiefly of nitrogen, without a trace of 

 carbonic acid. 



t CCCXCII. p. 364. \ CCCXCV. p. 705. 



Q Q 2 



