EESUMl:. 137 



similar evolute and involute forms ; but it is probable that Pcecilomorphus is 

 descended from the evolute forms of Am. Mercati, otherwise it would most likely 

 have by this time developed a greater number of auxiliary lobes. For some 

 reason or other the evolute forms gradually acquired a small spiral angle ; that 

 meant greater inclusion ; but greater inclusion meant a contraction of the space of 

 the body-chamber ; this has, therefore, been enlarged by expanding the middle of 

 the sides. That the expansion took place in the sides is shown by the increase in 

 the width of the superior lateral saddle, and the decrease of the siphonal saddle in 

 Pcecil. cycloides. 



Lillia has not abandoned the straight ribs ; but it has, for some reason, also 

 developed knobs on the inner area. Ludwigia has inherited these knobs, and has 

 lengthened them into stout primary ribs. In order to increase the breadth of the 

 sides in Ludwigia, growth has taken place in the inner part of the whorl ; and 

 therefore the inner margin has been pushed further towards the centre. Naturally 

 this would cause a lengthening of the ornamentation which adorned the inner 

 part of the whorl, while it affected but little that on the outer part. Hence this 

 change from knobs to ribs is merely the lengthening of the former ; l and in the same 

 way an increased number of auxiliary lobes should in time be developed to support 

 this increased breadth of side. The less developed forms of Ludwigia, — L. 

 obtusa ,&c. — retain some trace of the quadrangular whorls, and the broad siphonal 

 area with rudimentary furrows, and possess at the same time stronger — in the 

 young almost spinous — primary ribs ; the more developed forms — L. Baylii, 

 cornu, &c. — have compressed whorls, have entirely lost the ventral furrows, 

 have less pronounced primary ribs, and are more involute, — so that they show 

 a decided convergence towards Lioceras. The next step which they would 

 take in development would be to increase the number of their auxiliary lobes, 

 when they would be almost indistinguishable from Lioceras. However, it appears 

 that these forms died out. The dwarf Ludwigice {L. costosa, cornu, &c.) became 

 more highly developed at an earlier date than the others, and it is possible that 

 they may have sprung from an earlier-modified species of Lillia. 



Whether the Branch B or the Branch C left the parent-stem first is not, 

 with our present knowledge, easy to say. The latter have changed less alto- 

 gether ; and although they are already more developed in the James oni-zone than 

 is Hildoceras in the Commune-zone, yet they undergo little further modification 

 from that time, — so there exists a convergence in shape between them and 

 Hildoceras in the Lias-Oolite border-region. 



1 It is perhaps worth while to notice in passing how convergence may be brought about. If upon 

 the inner area two ribs happened to become joined they would thus produce a strong rib, similar in 

 appearance to the primary rib of Ludwigia. This is what sometimes happens in Grammoceras ; and 

 consequently we have at that time convergence in shape towards Ludwigia, but at other times 

 convergence towards Hildoceras or Lioceras. 



18 



