THE CEPHALOPODA 



127 



the whorl within, leaving only a very deep and narrow 

 umbilicus on each side. The shape of the whorl-section 

 thus comes to be that shown in Fig. 37 (right-hand 

 figure), and the outline may be divided into a peripheral 

 area, a pair of lateral areas, a pair of umbilical zones, 

 and an impressed area, of which the last is concave, 

 the rest more or less convex (see Fig. 49, e, for these 



FIG. 37. NAUTILUS ALBENSIS, D'ORBIGNY, CENOMANIAN, DERIVED 

 (CAMBRIDGE GREENSAND). 



Internal cast. Body-chamber at least missing. Umbilicus filled with 

 phosphatic matrix. Side view and front -view. (Natural size.) 

 (Original. ) 



terms). The peripheral area is known from the anatomy 

 of living species to be ventral, so that the umbilical and 

 impressed areas are dorsal. The ornamentation consists 

 of a series of slight corrugations which are essentially 

 growth-lines, and correspond to the aperture-margin, but 

 owing to the spiral coiling there has to be more rapid 

 growth on the ventral than on the dorsal side, and the 

 growth-lines are not parallel as they practically are in 



