611 
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 Lurystomites Kellogg’, 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 Lituitidze, 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 
Lituitide 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 Arietide,* the greater specialization 
and more complex ephebic development of the ontogeny in Am- > 
* Pp, 28-37. 
