THE CEPHALOPODA 



137 



the side which we shall presently recognize as ventral 

 being slightly concave : such a curvature is the opposite 

 to that of Nautilus, where the ventral side is convex. 

 This is expressed by saying that Tetrameroceras is endo- 

 gastric, Nautilus exogastric. A more striking difference 

 from both Orthoceras and Nautilus is the form of the 

 aperture, which is greatly narrowed in the centre, forming 

 a slit only 3 mm. wide. This widens at the ventral end 



Br. 



v 



a 



FIG. 40. TETRAMEROCERAS OBOVATUM (BLAKE), SILURIAN 

 (LOWER LUDLOW BEDS). 



a, Side view; b, apertural view, (xg.) (After Blake.) Br., Brachial 

 portion of aperture; H.S., hyponomic sinus; D, dorsal side; 

 V, ventral side. 



into an oval, at the dorsal end into a four-lobed expansion 

 Evidently the animal was permanently shut in its body- 

 chamber, and could only protrude two pairs of arms for 

 obtaining food by the larger four-lobed aperture, and the 

 swimming funnel by the other. The latter therefore 

 represents the hyponomic sinus and fixes the side 

 towards which it lies as ventral. In other species con- 

 temporary with this there may be openings for only 



