Mr. W. King on the Tetrabranchiate Cephalopods. 273 



to have existed at an early period of organic time ; and the oc- 

 curring of their testaceous coverings in every subsequent deposit, 

 together with their still inhabiting the Southern Ocean, prove 

 that they have survived to the present day. 



If we examine a specimen of the Pearly Nautilus which has 

 been longitudinally divided, its inner part will be seen to consist 

 of a number of cells or chambers, the partitions or dividing 

 plates of which have a small tubular perforation in the centre. 

 The animal or mollusk which inhabits this shell is, allowing for 

 some important differences, allied to that which tenants an ordi- 

 nary univalve : it occupies the outermost chamber ; but a por- 

 tion of its enveloping tegument or mantle, in the shape of a 

 slender membranous tube or siphon, accompanied with a vein 

 and an artery, passes through the whole of the chambers by 

 means of the tubular perforations*. This tube may be one means 

 of keeping the animal fixed in its place ; but the principal seem to 

 be two muscles, one on each side of the body, which are con- 

 nected with the lateral walls of the chamber. 



It is generally supposed that the chambers constitute u an ap- 

 paratus which renders the Nautilus nearly of the same specific 

 gravity as the surrounding water, and enables it to rise to the sur- 

 face of the sea, or sink to the bottom, simply by altering the ex- 

 tent of the surface exposed to the water by its soft partsf." 



Now the Ammonite or snake-stone, as it is popularly called, is 

 a shell which is constructed on the same general plan as the 

 Pearly Nautilus, and which it is generally admitted was fabricated 

 by a cephalopodous mollusk J. Specimens of the Ammonite are 

 quite common which show distinctly their inner part divided in 

 the same manner as the Nautilus, but in each of these genera 

 the siphonal sheath is differently placed, and the plates have a 

 different form. In the Ammonite the siphonal sheath is in con- 

 tact with the outer or dorsal part of the shell, whereas it is central 

 or nearly so in the Nautilus : and the edge of the plates, instead 

 of being plain and slightly sinuous, as in the last genus, alter- 



* The siphon appears to have been strongly protected in some fossil Nau- 

 tiluses both by an elongation of the tubular perforation and by additional 

 calcareous tubes (N. sipho and N. striatus, Buckland in ' Bridgewater 

 Treatise '). These parts, whether only one or both are known to occur in 

 a fossil, will be considered in the light of a siphonal sheath, and as such 

 will be termed in the following pages. 



f In my lecture, as it was originally delivered, Dr. Buckland's hypo- 

 thesis respecting the use of the siphon was examined. This hypothesis 

 having been shown to be untenable by the researches of Prof. Owen (vide 

 Hunterian Lectures on the Invertebrate Animals), I have thought it neces ■ 

 sary to cancel my original remarks, and to replace them by the above ex- 

 tract from Dr. Mantell's highly interesting ' Medals of Creation/ 



X The Ammonites, and all the other shells mentioned in this paper, are 

 placed in the tetrabranchiate division of the Cephalopods, in accordance 

 with the views of Prof. Owen. 



