130 PALEONTOLOGY 



straight across the centre to the opposite edge. The 

 measurement of a single whorl in the same direction 

 gives its height; at right angles to this is its thickness, 

 In an ammonite with strong ribs, such as Asteroceras, 

 this thickness will vary very much according as it is 

 measured at or between the ribs. The width of the 

 umbilicus is measured from whorl-suture to whorl- 

 suture along the same line as the diameter. These are 

 the most important dimensions, and it is their relative 

 proportions, rather than their absolute amounts, that are 

 of value for comparison of one species with another. As, 

 however, these proportions frequently alter in the process 

 of growth, the actual size of an ammonite measured must 

 always be stated. The four essentials of ammonite 

 measurement are, then (i) diameter; (2) height of last 

 whorl; (3) thickness of last whorl; (4) width of umbilicus. 

 The first may be stated in millimetres, the others as 

 percentages of the first. Thus for a particular specimen 

 of Asteroceras obtusum we express these dimensions thus: 



145, 36, 27-31, 41. 



The 27-31 refers to the difference of thickness between 

 and at the ribs. 



For comparison we give measurements of two allied 

 species, A steroceras redcarense : 



118-5: 36, 30-34, 37-5, 

 and Asteroceras olifex : 



88 : 34-5, 25-27, 38. 

 A glance at the three formulae shows us that A. redcarense 



