MACROCEPHALITES. 



n 



and the number of ribs from 24 to 50 (or 66), which two characters arc thus the 

 most important as specific indices. Lists of this sort show how useless it is to 

 run all the species together, but they serve as a means of separating those 

 species into which the genus is really divided. As the data obtained from the 

 columns D and E are the most important, a system may be adopted of displaying 

 them graphically and uniting with them other measures obtained from specimens 

 which show only those two satisfactorily. Such a scheme is shown in text-fig. 2, 



E D 



Fig. 2. — Diagram illustrating disagreement in detail of various Macrocephalites. 



in which the entries in column D are arranged along a vertical column increasing 

 downwards, and those in column E increasing upwards. The lines joining the 

 entries corresponding with the same specimens will thus be more nearly horizontal. 

 The divergence shown by these lines appears greater than could be expected in 

 a single species; in fact, we can see indicated several species characterised by (1) 

 small thickness and abundant ribs, shown by the upper lines; (2) great thickness 

 and few ribs, shown by the lower lines; (3) greater thickness and few ribs, shown 

 by lower lines sloping to the left, and possibly some more. These will be found 

 to correspond with the various species adopted. 



