123 PAL.EONTOLOGY 



gastropods, but radiating : in cephalopods they are there- 

 fore usually termed the radial lines. These lines are very 

 faint near the umbilicus, but become more distinct in the 

 outer part of the lateral areas, where they swing back- 

 wards and cross the peripheral area in a curve which is 

 concave forwards (towards the aperture). This is called 

 the hyponomic sinus (cf. Fig. 45, a'), and corresponds to an 

 embayment of the aperture occupied by the funnel or 



Siph uncle 



FIG. 38. NAUTILUS EXCAVATUS, J. DE C. SOWERBY, INFERIOR 

 OOLITE. 



Approximate median section (not accurately nifdian at the centre). 

 (Natural size. ) (Original.) 



hyponome, an organ by which the nautilus ejects water 

 from the mantle-chamber and so projects itself through 

 the sea. This hyponomic sinus is characteristic of most 

 Nautiloidea, though wanting in the primitive species of 

 Orthoceras. If the fossil is broken, the septa are seen to 

 be concave forwards as in Orthoceras, with the siphuncle 

 placed rather nearer the dorsal side (Fig. 37, right-hand 

 figure). On the cast the sutures are seen to run radially 



