XIII NAUTILOIDEA 393 



its walls are often thickened by the deposition of masses of calca- 

 reous matter, or by rings and radiating lamellae of the same 

 material. In position, the siphuncle is sometimes central, some- 

 times sub-central, sometimes (Ammonoi- 

 dea) marginal. In some cases its position 

 is believed to change during the growth of 

 the individual. The precise object served 

 by the siphuncle is at present unknown. 

 Some hold that it preserves the vitality of 

 the unoccupied chambers, by connecting 

 them with the soft parts of the animal; Fig. 253.— AjnmojiitesiCa- 



., , 11., c doceras) sublaevis Sowh., 



others have regarded it as a means for Keiiaway's Rock, show- 

 lightening^ the shell by the passage of some ing the marginal position 

 ^ . , ° , , -^ ^ * of the siphuncle (si). 



gas into the chambers. 



The initial chamber in Nautiloidea consists of an obtuse 

 incurved cone, marked on the outer surface of its posterior wall 

 by a small scar known as the cicatrix, Avhich msij be slit-like, 

 round, oval, or cruciform in shape. It has been held that the 

 cicatrix originally communicated with the protoconch or larval 

 shell, which probably dropped off as development proceeded. 

 In the Ammonoidea, on the other hand, there is no cicatrix, and 

 the initial chamber probably represents the protoconch, as seen 

 in the nucleus of many Gasteropoda. 



Sub-order i. Nautiloidea. — Shell straight, bent, or coiled, 

 aperture simple or contracted; siphuncle often narrowed by 

 internal deposits, position variable ; septal necks short, usually 

 directed backwards ; septa concave towards the aperture ; initial 

 chamber conical, with a cicatrix on the posterior wall. 



The Nautiloidea, of which Nautilus is the sole living repre- 

 sentative, date back to the Cambrian epoch, and attain their 

 maximum in the Silurian and Devonian. At the close of the 

 Palaeozoic era, every family, with the sole exceptions of the 

 Orthoceratidae and Nautilidae, appears to have become extinct. 

 The former disappear with the Trias, and after the lapse of the 

 whole Secondary era, Aturia, a form closely related to Nautilus, 

 makes its appearance. 



(a) Retrosiphonata : septal necks directed backwards. 



Fam. 1. Orthoceratidae} — Shell straight or slightly curved, 

 aperture simple, body-chamber large ; siphuncle cylindrical, 



1 The classification is that of Foord, Catal. Fossil Cephal. Brit. Mus., 1888. 



