MOLLUSCA. CEPHALOPODA 267 



small collar-like part projects in front of the septum, but 

 the main part of the septal neck extends backwards. 



The form of the sutures varies considerably in different 



genera and is of great importance for systematic purposes. 



The central part of each septum is flattened or slightly 



undulose, but the edges become folded or even frilled, 



often giving rise to very complex sutures ; by this means 



greater support is afforded to the outer tubular part of 



the shell than is the case in the Nautiloids where the 



sutures are simple. The portions of the suture which are 



convex towards the mouth of the shell are termed saddles 



(fig. 107, s), while the intervening concave portions are 



known as the lobes (I). In many forms the lobes and 



saddles exhibit secondary foldings, which may be slight, 



producing merely a denticulate pattern, or may be deep 



and provided with other smaller foldings, giving a folia- 



ceous appearance to the suture. The lobes and saddles 



are similar on the two sides of the shell : commonly there 



is first the external lobe (fig. 107, 1) at the external margin, 



then the superior and inferior lateral lobes on the sides of 



the whorl (1 1, 2 I), and near the inner margin other lobes 



known as auxiliary lobes (al) may occur; on the internal 



margin (opposite to the external lobe) is the internal lobe. 



The saddles are arranged in a similar manner ; there are 



the external saddle (s), the lateral saddles (Is, 2 s), and 



auxiliary saddles (as). The external lobe is often divided 



by a median saddle (as in fig. 107). The form of the 



successive sutures remains almost constant on the adult 



part of the shell, but on the younger parts the sutures are 



less complex. The suture of the first septum may be 



straight or only slightly curved, as in the Nautiloids ; or 



it may show a broad external saddle ; or a narrow external 



saddle with a lateral lobe on each side. In the second 



