NAUTILOIDEA. 



845 



the siphuncle is variable in position, but is usually placed between 

 the centre and the convex side ; and the surface is commonly 

 adorned with tubercles or ribs. The 

 genus ranges from the Silurian to the 

 Carboniferous. 



In Hercoceras the shell is discoidal, 

 with contiguous whorls and a wide um- 

 bilicus. The aperture is contracted ; the 

 sutures are simple ; the siphuncle is sub- 

 marginal and external ; and the surface 

 is adorned with transverse striae or a 

 row of projecting tubercles. The two 

 known species are Silurian. 



In the genus Nautilus itself (fig. 

 759) the shell is involute or discoidal, 

 consisting of a few whorls coiled into 



a flat spiral, the volutions being in contact, and the last turn of 

 the shell commonly more or less concealing the previous ones. In 

 the young condition of the shell a central vacuity exists behind the 

 conical initial chamber ; but in the typical forms of the genus this 

 becomes ultimately filled up by a secondary deposit of shell, or 

 becomes concealed by the later volutions. The body-chamber is 

 capacious, and the aperture is simple and has a ventral sinus. The 



Fig. 758. — Gyroceras ornatum. 

 Devonian. 





Fig. 759. — Nautilus Danicus. Upper Cretaceous. 



\A 



i 



J 



septa are concave forwards, and the suture-lines are simple or may 

 show a slight ventral or dorsal "lobe." The siphuncle is subcen- 

 tral in position, or may be placed between the centre and either 

 the ventral or the dorsal margin, and it is never contracted by 

 internal deposits ; while the septal " necks " are always short. 

 The surface may be quite smooth (Lcevigati), or adorned with 

 transverse striae (Striati), or with markedly distinct ribs {Radiati). 

 The genus Nautilus has been split up into a number of minor 



