126 PAL/EONTOLOGY 



Through the central perforations of the septa ran a 

 living cord, the siphuncle, through all the gas chambers 

 back to the protoconch. While the animal was growing 

 new shell continually was being added to the aperture or 

 " mouth " of the body-chamber ; while at intervals the 

 animal moved forwards in its body-chamber, and secreted 

 behind it a new septum. Thus any given septum was 

 formed at a rather later time than the part of the external 

 shell with which it is in contact a fact that must never 

 be forgotten. The line of junction of the edge of a 

 septum with the external shell is termed the suture (more 

 precisely the septal suture, for in coiled cephalopods the 

 term suture is also used in the same sense as in gastropods, 

 for the line of external contact of the whorls). The 

 septal sutures are invisible externally, but when the shell 

 is removed the internal cast shows them. 



The ornamentation of Orthoceras is usually of a very 

 simple character; consisting of growth- lines parallel to 

 the margin of the aperture usually only fine striae, at 

 other times with stronger rings at intervals. The actual 

 aperture of the body-chamber is rarely preserved (the 

 thin shell, unsupported by septa, being easily crushed) ; 

 but in a few perfect specimens the aperture is seen to be 

 a simple circle, which may or may not have just behind 

 it a circular constriction (Fig. 36, b). 



2. Nautilus radiatus is a fairly common species in 

 the Lower Chalk, and the following description applies 

 to it, but, with slight alterations, would serve for many 

 other species. It is a shell symmetrically coiled in 

 a plane-spiral, so tightly that each whorl nearly conceals 



