Values of the Stages of Growtn and Decline. 883 
There were two stages in the old age period among Ammonoids : 
the first of these can be designated as the Clinologic! stage. This 
immediately succeeded the ephebolic period and during its continu- 
ance the nealogic and ephebolic characteristics underwent retrogres- 
sion, Ornaments, spines, and sutures degenerated and lost their 
angularity, the ribs of pile, and often the keel and channels, when 
the latter were present, became less prominent, and before this stage 
closed the whorl itself sometimes decreased, showing that degenera- 
tion in the growth force of the animal had taken place. Similar 
phenomena can be easily observed in other departments of the 
animal kingdom, notably in man, whose habits tend to preserve life 
until he has attained extreme age. During this period there is a 
steady loss of the differential characters acquired during the stages 
of progressive growth and there is a tendency to resume the propor- 
tions and aspect of the earlier nealogic stages. | In man, baldness 
of the head, loss of teeth and resorption of the alveoli, loss of the 
calves, rotund stomach, and the return of early mental peculiarities, 
are phenomena of similar import. 
The last changes in the ontology of the animal may be termed 
the Nostologic stage,? and during this stage these tendencies reached 
their highest expression. Among Ammonoids the ornaments were 
all lost by resorption, the whorl became almost,as round and 
smooth as it was in the silphologic stage, and in extreme cases 
it was separated from the next whorl, leaving a perceptible gap. 
This almost complete reversion to the aspect of the silphologic 
Stage can of course only occur in animals which attain an extreme 
age, 
The correlations of Clinology are exact, and indicate the changes 
which may be expected to occur in the same group whenever de- 
graded or aberrent species can be traced ina more or less continuous 
Series of graded modifications starting with any given normal form, 
Many such series have been traced, and these are recognized now by 
all paleontologists as genetically connected. They began with 
normal, close coiled, ornamented, shells, the descendants were 
smaller, showing a tendency to be less involved by growth, to lose 
their ornaments, and simplify the outlines of the sutures, though 
* Kiva, to incline downwards. 
* Néozog, a return. 
