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noticeable and one of the most persistent characteristics of the 

 general morphology of the order. 



Nevertheless, in * every example of uncoiled phylogerontic shells 

 the impressed zone tends to become less and to disappear, obeying 

 the same law as among the Nautiloids. There is, however, a dif- 

 ference in its behavior, which is at first rather confusing. Involute 

 shells may have free gerontic volutions and in these the zone does 

 not appear to have, as a rule, so deep a hold upon the organization 

 of the Ammonoids that it does upon many of the Nautiloids. 

 For example, in deeply involute Scaphitoid shells there may be free 

 living chambers in the gerontic stage and the zone diminishes 

 greatly, almost disappearing on the edge of the aperture. In crio- 

 ceran and baculites-like forms, however, it does not appear to per- 

 sist to any marked extent upon the dorsum beyond the cessation of 

 contact in the young whorl. 



There are no examples in the history of its retrogression which 

 can be compared with the persistency exhibited in a number of 

 Nautiloids. For example, in Eurystomites Kelloggi, Ophidioceras 

 and others the impressed zone, although it may not be present in 

 the nepionic stage before contact, and very shallow after contact, 

 nevertheless persists in the gerontic stage. Although showing a 

 tendency to disappear and finally vanishing at the aperture, the pro- 

 cess is slow, and it has obviously made a strong impression upon 

 the organism. 



What has previously been said of the degenerative characteristics 

 and degenerative series of the Nautiloidea may be of some assist- 

 ance in clearing up this apparent anomaly. The phylogerontic 

 transformations of the Lituitidse, as stated above, are the only ones 

 among Nautiloids that can be compared with any of the completely 

 uncoiled retrogressive series of Ammonoids. Although in the 

 Lituitidae the impressed zone is a mere contact furrow of slight 

 extent and obviously transient development, nevertheless they serve 

 as a comparative standard to show how much more complete the 

 degenerative changes are among the Ammonoids than among Nauti- 

 loids. If the observer studies any species of Ammonoid in the 

 gerontic stage the same morphic law becomes apparent. As I have 

 tried to show in Genesis of the Arietidce* the greater specialization 

 and more complex ephebic development of the ontogeny in Ara- 



* Pp. 28-37. 



