THE CEPHALOPODA 143 



lasting rather longer. It is thus the final stage of 

 uncoiling (baculicone}. 



These three forms illustrate the reversal of the process 

 which led from orthocone to ophiocone. There are many 

 indications that the closely-coiled forms which precede 

 them in the Jurassic and Cretaceous are themselves 

 derived from sphaerocones, so they may be termed 

 serpenticones. Thus we have a double series, one of 

 increasing curvature (anagenesis] and one of decreasing 

 curvature (catagenesis}. 



Sphaerocone 



./ Ni 



Ophiocone Serpenticone 



/" \ 



Gyrocone Criocone 



Cyrtocone Toxocone 



/ \ 



Baculi 



Orthocone Baculicone 



Such a double series forms an evolutionary cycle. It 

 should never be possible to confuse the corresponding 

 stages of the upward and downward series, because 



(1) the ontogeny of a perfect specimen should show some 

 trace of the stages earlier in the series, and if this fails 



(2) there are other features, such as the sutures, which 

 have their own independent anagenesis and catagenesis, 

 and their stages will never exactly correspond with the 

 coiling stages. Thus we find degenerate suture-lines in 

 Cretaceous serpenticones, but not in criocones, etc. This 

 is an example of the principle of iyreversibility in evo- 

 lution, according to which an organism which reverts to 



