134 INFERIOR OOLITE AMMONITES. 



appear to have been forced by circumstance or otherwise to work towards the same 

 end, though by different methods, at different times, and with varying degrees of 

 success — would seem to be capable of being reduced to a series of definite laws, 1 

 like the following : 



I. Towards the attainment of greater involution. 

 To attain this end was required — 



a. An increase in the breadth of the whorl. 



As compensation for the increase in breadth there came — 



b. A decrease in comparative thickness? and a lessened number of ivhorls. 

 In consequence of the decrease in thickness there was — 



c. Less space for furrows each side of the carina. And therefore — 



d. The furrows were gradually lost, or only reappear sometimes through 



reversion. 

 To support the increased side-surface of the whorl we find — 



e. More complicated sutures are developed. 



In consequence of the decreased width of the ventral area, and to compensate 

 for the increased growth of the lateral lobes — 



/. A proportionate decrease of the length and importance of the siphonal 

 lobe in comparison with the superior lateral, and especially in com- 

 parison with the whole suture-line. 3 

 In consequence of the increased occlusion of the preceding whorl (which 

 meant increased breadth of the side) — 



g. An ever larger number of auxiliary lobes is developed. 



II. A change from ribs straight on the lateral area to ribs projected forwards 

 on the inner part of the whorl and then recurved, i. e. to sigmoidal-shaped ribbing. 



Most of the above changes can be summarised in the following : 



III. The assumption (by the descendants), at an ever earlier age, of the 

 characters of their adult progenitors. 4 



It is not to be supposed that all these changes occurred in regular and precise 



1 I do not wish to say that these laws will apply to other families. All that I attempt to do at 

 present is to demonstrate that the tracing out of the descent of the Hildoeeratidse seems to show that 

 their mode of development is capable of being expressed by these laws. 



2 A quadrangular whorl, like that of Hildoceras, has a siphonal area very broad when compared 

 with the lateral area ; in Lioceras, even if tbe siphonal area were as broad, it would, when compared with 

 the lateral area, be but insignificant in extent. The increase of the lateral lobes, aud decrease of tbe 

 ventral, may be partly accounted for in the same way ; but they were, in addition, influenced by the 

 changes in the whorl-area, and by having to provide support for a different place. 



3 Compare the highly -developed Hyperlioceras with the little-developed Hildoceras. 



4 Each generation must apparently bave not only taken on the characters of its ancestors at a 

 very slightly earlier age, but must have also added something of its own to the process of change ; 

 that is to say, that it must have exaggerated, or rather helped to intensify, so far as circumstances 



