132 PALAEONTOLOGY 



is thicker, A . olifex thinner than A . obtusuw, while both 

 are less widely umbilicate. 



If we place the specimen so that the aperture faces us 

 (apertural view, Fig. 39, b) we see the shape of the whorl 

 in cross-section. It is somewhat elliptical, and the outline 

 may be divided into six parts, corresponding to six regions 

 on the surface of the whorl : (i) the somewhat flattened 

 peripheral or external area or venter, which bears a broad 

 rounded keel in its middle line; (2) and (3) the lateral areas, 

 one on each side, slightly convex but with a general 

 direction approximately parallel to the plane of symmetry; 

 (4) and (5) the inner or umbilical areas ; one on each side, 

 which slope inwards to the whorl-sutures ; and (6) the 

 concave impressed area, invisible except when the whorl is 

 detached from the shell, lying between the two whorl- 

 sutures and in close contact with the next inner whorl. 

 The ratio between the height of this overlapped part of 

 an inner whorl and the height of the outer whorl which 

 overlaps it is the amount of indentation of the outer whorl ; 

 while the ratio between it and the total height of the 

 whorl of which it forms part is the amount of inclusion 

 of the inner whorl. In A. obtusum the former is about \, 

 the latter about | (the height of the outer whorl being 

 about double that of the next inner). These ratios are 

 low when the umbilicus is wide (as in Asteroceras], but 

 they increase as it becomes narrower. 



The spiral line along which the peripheral area meets 

 the lateral area is called the peripheral margin ; that on 

 which the lateral and inner areas meet is the umbilical 

 margin (as for descriptive purposes the umbilicus is taken 



