136 INFERIOR OOLITE AMMONITES. 



genus, the parent-forms of the three stems. But there exists a great check to 

 this desire, namely, the fact that the development or branching-off of the different 

 stems took place at such very different periods. If we look at the Margaritatus- 

 zone, we see that the three stems were very fairly distinct, and we can very easily 

 perceive a wide difference between Am. boscensis and Am. algovianus ; but between 

 Am. boscensis and Hildoceras there is less difference, because they both represent 

 about a similar state of development from the parent-form. This is where the 

 error in regard to grouping is likely to come in, and this is where the greatest 

 difficulties lie. Ludwigia, again, is more nearly alike in general aspect to Lioceras 

 than to Hildoceras ; and this is simply because Lioceras has been stationary in deve- 

 lopment, while Ludwigia has been' gaining upon it until it nearly overtakes it. 

 But if our classification is to be by descent, and not solely by similarity of aspect, 

 we must place Ludicigia and Lioceras apart into two separate branches in accord- 

 ance with their descent. 



The Branch A is, without doubt, the oldest or least changed of the three. 

 The ventral furrows are retained by Hildoceras, Lillia, and Poexilomorphus ; 

 but they are, on the other hand, rudimentary only in a few forms of 

 Ludwigia, and are, in fact, entirely absent from the majority. The quadrangular 

 whorls with evolute form are common to Hildoceras and Lillia ; but in each genus 

 there is a tendency to modify both these features. The simple suture-line, con- 

 sisting of a large superior-lateral lobe, small inferior-lateral lobe, and about one 

 auxiliary, is common with but little modification to Hildoceras, Lillia, and even the 

 much later Pmcilomorphus. This lobe-line is slightly more developed than that of 

 Am. algovianus. In the Arietitan parent-form the siphonal lobe was longer than the 

 superior-lateral ; in these genera it has decreased somewhat in size, but since they 

 still retain the very broad ventral area, and as they have made but little progress 

 towards inclusion, 1 it is not much affected. In Am. algovianus we have ribs 

 straight on the lateral area; but Hildoceras shows us a transition state in the 

 matter of ribbing, namely, from the straight ribs of Hildoceras Douvillei to the 

 strongly curved ribs of Hildoceras bifrons. In the matter of increased inclusion the 

 specimens of the latter species from the Cotteswold Sands show a beginning ; 

 they also exhibit a tendency to develop more auxiliary lobes to accompany the 

 change ; while they are at the same time more compressed. 



Ammonites Mercati still possesses ribs fairly straight on the lateral area ; 

 Pcecilomorphus shows a complete change, and also has a somewhat more deve- 

 loped suture-line. This genus exhibits very peculiar modes of growth. We 

 have strongly involute forms with great thickness, while the evolute varieties are 

 more compressed — although these rules do not hold good always. In most cases 

 the proportionate height of the whorl remains the same. Am. Mercati shows 



1 Except Poecil. cycloides : but see next page. 



