M4 P 



the condition of one of its remoter ancestors never does 

 so entirely : it retains features acquired by its inter- 

 mediate ancestors. It should thus always be possible to 

 distinguish degenerate from primitive forms. 



As with the anagenetic series, so with the catagenetic : 

 the same stages are passed through by different stocks at 

 different times. Uncoiling forms, including a baculicone, 

 occur as early as the Noric (Upper Triassic), where 

 they are contemporaneous with the last orthocones ; but 

 it is not until the Cretaceous that they become abundant. 



Just as the early ophiocones often show a tendency to 

 revert to the straight form in old age, so the uncoiling 

 forms show a tendency to coil again in old age, producing 

 hook-like body-chambers. Ancyloceras ends thus after a 

 criocone stage, Macroscaphites after a serpenticone stage, 

 while Scaphites passes direct from sphaerocone to this 

 hook. Hamites (initial stages unknown) makes two 

 hook-like bends. All these are Cretaceous (Fig. 41, g-j), 

 but in the Jurassic period CEcoptychius had the form of 

 Scaphites (but different ornament), and there were a 

 number of sphserocones which showed a tendency to 

 uncoil in old age (scaphitoids). Sometimes there are 

 signs in ontogeny of uncoiling followed by re-coiling. 



All these forms so far described are bilaterally sym- 

 metrical, but there are a few asymmetrically-coiled 

 forms (turricones), resembling sinistral turreted gastropods, 

 mainly Cretaceous (e.g., Tnrrilites, Fig. 41, e) but with one 

 example in the Noric; and one extraordinary form 

 (Nipponites) which can best be described as a three- 

 dimensional zigzag. By analogy with the gastropods, 



