THE CEPHALOPODA 133 



to include the inner area of the outer whorl, but not for 

 purposes of measurement). In Asteroceras these margins 

 are not sharply denned, as they are in some ammonites. 



The ornament of Asteroceras consists of a series of 

 strong ribs, which start at the umbilical margin and 

 run almost straight outwards until near the peripheral 

 margin, where they bend decidedly forwards and die 

 away. Between their ends and the median keel, the 

 periphery forms a shallow groove on either side. The 

 forward bend of the ribs shows that the aperture instead 

 of having a hyponomic sinus, as in Nautilus, has a 

 median projection or rostrum (cf. Fig. 45, /). 



The ribs are well spaced- out, their distance apart being 

 proportionate to the height of the whorl ; thus in the last 

 whorl of the specimen figured it gradually increases from 

 8 mm. to 1 6 mm. (as measured in the middle of the 

 lateral area). Careful examination of the inner whorls 

 shows that the ribs there are not so smooth as in the 

 outer whorls : each is thickened to form a tubercle about 

 the middle of the lateral area. This ammonite, as it grew 

 up, suffered a degeneration (catagenesis) of its ornament, 

 from more to less complex. 



The ribs and tubercles are not thickenings of the shell, 

 but corrugations, and therefore show as well on the cast 

 as when the shell (or test) is present. 



The septal sutures (Fig. 39, c} are very different from 

 those of Orthoceras or Nautilus : they are thrown into a 

 series of forward and backward curves, each of which is 

 further puckered up or frilled. The forward curves 

 (convex towards the body-chamber) are called saddles, the 



