THE DECADENCE OF AMMONITES 



401 



and Cochloceratidce not only show unrolled forms, but their 

 lobes and saddles undergo a corresponding retrogressive sim- 

 plicity by being reduced to six in the former and four in the 

 latter family. The transitional stages in the process of unrolling 

 can be traced from the normal spiral of Polycyclus (fig. 2) to the 

 partly unrolled spiral of Choristoceras (fig. 3), the pulled-out 

 turret-shell of Cochloceras (fig. 4), and finally to the straightened- 

 out, rod-like Rliabdoceras (fig. 5). 



The Phyllocampyli was the only sub-order of Ammonites to 

 survive through the Jurassic and Cretaceous periods. It has 

 been subdivided into the Leptocampyli and Pachycampyli, both 

 of which contain species showing the phenomenon of unrolling. 

 In the Leptocampyli the transitional forms may be typically 

 represented in gradation from the normal spiral of the Tithonian 

 Lytoceras (fig. 6) to the partially unrolled Macroscaphites (fig. 7) 

 and Scaphites (fig. 8), the straightened, hook-like Anahamulina 

 (fig. 9), and Ptychoceras (fig. 10), in which the straight rod is 

 bent upon itself, finally leading to the slightly curved or 

 perfectly straight forms of Baculites (figs. 11 and 12). Even 

 Baculites, in its infantile stage, starts as a closely coiled shell. 

 The affinity of all these genera with each other is demonstrated 

 by their similar suture lines, although these became much 

 simplified in the straighter forms. 



Whilst these anomalous forms were all developed in the 

 Cretaceous period, there is a family of the Pachycampyli — the 

 Spiroceratidce (belonging to the group of Morphoccratidce) — which 



