8 3 o 



DIVISIONS OF THE CEPHALOPODA. 



shell are in contact with one another, in which case the coils may 

 lie in one plane {Nautilus, Ammonites, &c.), or they may pass ob- 

 liquely round an axis (Turrilites). In the discoidal types (such as 

 Nautilus and the Ammonites) the last volution may more or less 

 completely conceal the preceding turns of the spiral. In other 

 cases the earlier coils of the shell remain visible, and the shell 

 becomes umbilicated. 



The surface of the shell in the Tetrabranchiates may be nearly 

 smooth, or may show nothing more than delicate lines of growth. 

 In other cases the surface may be more or less conspicuously 

 adorned with various kinds of sculpturing, which may have a direc- 

 tion corresponding with the long axis of the shell, or may be at right 

 angles to this. Thin sections show that the shell is composed of 

 two principal layers, of a different nature. The outer layer forms a 

 thin porcellanous stratum, which is lined throughout by a thicker 

 nacreous layer, which alone forms the septa between the air- 

 chambers. 



The " septa," or partitions between the successive air-chambers, 

 vary greatly in number in different types of the Tetrabranchiates. 



Fig. 742. — Diagram to illustrate the position of the siphuncle and the form of the septa in 

 various Tetrabranchiate Cephalopods. The upper row of figures represents transverse sections 

 of the shells, the lower row represents the edges of the septa, a a, Ammonite or Baculite', b b, 

 Ceratite; c c, Go?iiatite ; d d, Clymenia; e e, Nautilus or Orthoceras. 



Whatever their number may be, they are usually placed in each 

 individual at approximately uniform distances apart in each succes- 

 sive portion of the shell ; but the last two or three septa are com- 

 monly closer to one another than the rest, probably in consequence 

 of an impairment of the vitality of the animal with advancing age. 

 Up to a certain period of the animal's life, new chambers are suc- 

 cessively formed as the increasing size of the body necessitates the 

 acquisition of more room, but the process by which the animal 

 moves forwards prior to the development of a fresh septum is not 

 perfectly understood. The edges of the septa, where they become 



