612 
monitinee is attended by a correspondingly intensified series of 
degenerative changes in the gerontic stage. 
Considering the nature and extent of these retrogressive changes 
in both ontogeny and phylogeny, one ceases to be astonished that 
the impressed zone disappears quickly in any individual and begins 
to wonder at the conservative power of genism which preserves the 
close coil of the nepionic stage as a definite record of the derivation 
of such straightened out shells as Baculites. I have tried to account 
for this by supposing that the young of even these degenerate forms 
had similar habits as those of their ancestors, or were specially pro- 
tected. The former supposition may be the true one, since it is 
entirely in accord with the facts that the tendency to degeneration 
should not necessarily take effect upon the development of the 
earlier stages, but the latter can hardly be true. 
The retrogressive forms have usually slenderer shells than the nor- 
mal forms, and there is no evidence that they possessed any special 
pouches or contrivances for the protection of the young. 
It is interesting in this connection to notice the results of an ex- 
tended research made by Dr. F. J. Pompeckj upon these extra- 
ordinary Ammonitinz. His observations include about all of the 
more remarkable distorted fossils of this kind from the Carboniferous 
to the Cretaceous, especially those which have contracted apertures 
and exhibit connections with normal forms. Dr. Pompeckj arrives 
at the conclusion that the living chambers having such peculiarities 
are one and all to be classed as senile characteristics. 
His conclusions are as follows : 
**z. Die Bildung ‘anormaler’ Wohnkammern ist nicht mit 
Resorptionserscheinungen verbunden; Resorptionserscheinungen 
sind an den Ammonitenschalen iiberhaupt nicht nachzuweisen. 
‘¢2, Ein Ammonit mit ‘ anormaler’* Wohnkammer ist fast aus- 
nahmlos als vollkommen ausgewachsen zu betrachten. 
“3. Der ‘anormalen’ Wohnkammer gehen weniger verinderte 
Wohnkammern voraus und schliesslich in den Jugendstadien solche, 
die.in vollkommen regelmassiger Spirale gewachsen sind; man 
darf daher bei der Beschreibung von Ammoniten nicht eigentlich 
von einer anormalen Wohnkammer sprechen, sondern von einer 
anormalen letzen Wohnkammer des ausgewachsenen Individuums. 
**4. Die ‘anormalen’ Wohnkammern der Ammoniten sind 
nicht auf sexuelle Unterschiede zuriickzufiihren. 
*¢5. Die ‘anormalen’ Wohnkammern und die mit denselben 
