THE CEPHALOPODA 149 



changes through which the order passes, but the following 

 may be given : 



(i) The protoconch is calcareous and is never shed; 

 (2) the septa are like those of nautiloids in the most 

 primitive families, but soon become folded and eventually 

 frilled at the edges, so that the suture-lines attain a very 

 high degree of complexity ; (3) the siphuncle migrates to 

 one of the margins, in the great majority to the periphery ; 

 (4) the backwardly directed septal necks (retrosiphonate) 

 soon become replaced by forwardly projecting septal collars 

 (pvosiplwnate) ; (5) the hyponomic sinus is present in most 

 Palaeozoic genera, but afterwards disappears, and may 

 be replaced by a rostrum. 



3. The Dibranchiata vary so much in shell-structure 

 that it is difficult to make any statement that shall apply 

 to the whole order. The main feature is the general 

 subordination of the chambered shell (phragmocone) to 

 other skeletal structures, resulting in a great delay of 

 the tendency to coil. The septa are simple, the siphuncle 

 more or less marginal and retrosiphonate. 



Among the large number of Palaeozoic nautiloids we 

 may choose a few for mention. 



(1) Genera retaining the straight form of Orthoceras, 

 but distinguished from it by peculiar features of the 

 siphuncle. In Endoceras (Ord.-Sil.) the siphuncle is nearly 

 half the width of the shell and the septal necks are 

 funnel-like and fit in to one another,~forming a continuous 

 tube. In Actinoceras the siphuncle is central, and swells 

 out in each gas-chamber, but is constricted as it passes 

 through the septa. Internal casts of the siphuncle of a 

 gigantic species of this type resemble a row of vertebrae 

 (Huronia vertebralis) when weathered out from the Ordo- 

 vician limestone of Drummond Island, Lake Huron. 



(2) Orthocones or cyrtocones which acquire peculiar 



