382 INFERIOR OOLITE AMMONITES. 



lobe with intra-axial, inequicellate, endo-brachysceles terminal lobule, the lobe 

 with axial, equicellate, isosceles terminal lobule would be a necessary phase. 



Primarily, then, the differences between the septa of the various series is 

 whether the symmetrical, axially lobulate lobe was persistent throughout 

 catagenesis as in the qiiadrifida-stock, or whether the asymmetrical, intra-axially 

 lobulate lobe makes its appearance early in catagenesis, or even before the period 

 of phyletic decline had begun. It is the possibility of the symmetrical passing into 

 the asymmetrical lobe at any stage in phyletic development which makes it so 

 difficult to correctly and concisely define the differences between the various series. 

 In all cases it is necessary to insist on the comparison of septa of species in the 

 same stages of phyletic development, — for instance, of spinous species with spinous 

 species, and so on ; or, in other words, in considering differences of septation 

 morphic equivalents must always be compared. 



In order to facilitate the study of the following classification, skeleton outlines 

 of the various types of superior lateral lobe have been prepared by actual tracing 

 of the general outline in each case. The denticulations have been disregarded ; 

 and all that it is sought to illustrate is the disposition of the lobules which, when 

 well developed, constitute the somewhat cruciform arrangement so characteristic 

 of all these fossils, or that asymmetrical departure from such arrangement which 

 is equally striking. In the main the cause of the departure seems to be the 

 necessity for greater support of the area beyond the superior lateral lobe 

 consequent on the acquirement of less gibbous whorls. 1 It will be noticed in the 

 first place that in the different stocks these flatter whorls are acquired earlier or 

 later in relation to other characters — say spinosity ; that the asymmetry of L 2 is 

 also acquired at different times in relation to the flatter whorls, or it may not be 

 acquired at all. However, the difference in time of acquirement, reckoned in 

 regard to other characters, becomes a point of distinction between two forms, and 

 between the forms of two stocks ; and further, the differences give rise to a 

 different degree of asymmetry in correlation with the same degree of some other 

 character. 



As it is by no means easy to make this intelligible to those who have not tried 

 to study the matter from the same point of view, it may be desirable to attempt 

 the following method of explanation. Let x = the degree of spinosity, y = the 

 degree of gibbosity, z = the degree of asymmetry of L ; z at starting being practi- 

 cally symmetrical. Then the characters of the first species (a) may be represented 

 by the formula x-\-y -\- z. But the characters of the second species (b) compared 

 to those of a must be written f + I + z > and the characters of a third species (c) 



1 See p. 138. 



2 The following abbreviations will be used ; they are not new to Ammonite literature : — L = 

 superior lateral lobe ; I = inferior lateral lobe. 



