MOLLUSCA. CEPHALOPODA. 



173 



is constricted. The line where the edges of the septa 

 unite with the shell is known as the suture (fig. 50), 

 obviously this will only be seen when the shell is removed ; 

 but the fossil forms frequently occur as casts and in these 

 the sutures are admirably shown. The form of the sutures 

 varies considerably in different genera and is of great 



FIG. 50. A, suture of an Ammonite (Parkinsonia parkinsoni) from the 

 Inferior Oolite. B, Suture of Ceratites nodosus, from the Muschel- 

 kalk. I, external lobe ; II, 2 1, superior and inferior lateral lobes; al, 

 auxiliary lobes ; , external saddle ; 1 s, 2s, superior and inferior lateral 

 saddles ; as, auxiliary saddles. In each case the straight line on the 

 left represents the position of the siphuncle, and the curved one on 

 the right the line of contact with the next whorl. 



importance for systematic purposes; in some cases they 

 are straight or only slightly curved, in others they are 

 folded many times, the portions which are convex towards 

 the mouth of the shell being termed the saddles (fig. 50 s), 

 whilst the intervening concave portions are known as the 

 lobes (I). In many forms the lobes and saddles exhibit 

 secondary foldings, these may be slight, producing merely 

 a denticulate suture, or they may be deep and provided 

 with other foldings, giving a foliaceous appearance to the 



