20 



rolled round upon itself in one plane. Now if you try to consider 

 what variations of form you can produce with a flexible cylinder 

 (keeping to regular curves), and if you write down all these varia- 

 tions as they occur to you, I think you will find when you look over 

 your list that you will have been outwitted by Nature. 



Of course you can make your cylinder straight ; — Nature thought of 

 that, for this form is the common Baculite of the Chalk-marl (fig. 

 13); or you can make one or two bends in the straight cylinder, 



Fig. 13 



Baculites baeuloides, after D'Orbigny. 



in this you have the Hamite (fig. 14); or you can place the straight 

 bends in contact, — what is this but the Ptychoceras ? or you can twist 



Fig. 15. 



Turrilites Schcuchzerianus, after D'Orbigny, a common Lower 

 Chalk form, sometimes a foot and a half in length. 



your cylinder like a spiral, with the curves touching, and you produce 

 the Turrilite (fig. 15), or with the curves not touching, as in the 

 Helicoceras ; or you can make a bow and so realize Toxoceras ; or you 

 can form an Ammonite, lengthen out one end, and bend over the 



Fig. 16. Fig. 17. 



Ancyloceras gigas, from the Lower Greensand, Crioceras Comuelianus, from the Lower 

 sometimes a couple of feet in length. Greensand, after D'Orbigny. 



