MOLLUSC A— CEPHALOPODA. 2 1 



The Sutures. — This term is applied to septal edges, exposed on 

 the removal of the shell. In Nautiloids they are simple, straight 

 or slightly undulating lines which encircle the solid internal mold 

 at nearly regular or slightly increasing intervals. In some special- 

 ized Nautiloids, and occasionally in the straight or curved forms 

 as well, regularly disposed forward loops or saddles and backward 

 bending loops or lohes occur, these becoming very pronounced in 

 some cases (Figs. 1341-1344). In the Ammonoidea the sutures 

 are characterized by well developed lobes and saddles; these are 



Subventraa 



Propiov^entran 



Extracentrov/entrdn 



Ce-ntroventra-Ti 



Intra, cent rove Tit ran 



Ve-ntrocentren. 



Centren. 



Dorsocentren 



Intra-centrodorsan 



CeWtrodorsa-Ti 



Extracentrodorsan 



Propiodor 5dn 



Subdorsan 



Fig. 123 1. Diagram illustrating method of naming location of 

 siphuncle. (After Hyatt. ) 



entire in the primitive types and the young of more specialized 

 forms {goniatitic sutures, Figs. 1388-1396; Goniatites of authors 

 generally) ; notched at the bottom of the lobe {ceratitic suture, 

 Figs. 1401-1404; Ceratites of authors), or notched and lobed on 

 both saddle and lobe (ammonitic suture. Figs. 1408-1469; Ammo- 

 nites in the broad sense of authors). In general the degree of 

 complexity of the suture is an index of the stage reached in devel- 

 opment of both individual (ontogenetic) and race (phylogenetic), 

 but owing to a process of retardation in development or degenera- 

 tion of this especial feature, descendants of a highly specialized type 

 and occurring in a late horizon, may have a very simple type of 

 suture. This is the case with the Cretacic Pseudoceratites, Ammo- 



