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 (retro sip Jionate] 

 soon become replaced by forwardly projecting septal collars 

 (prosiphonate] ; (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 



