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monitinae 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 Ammonitinse. 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 : 



" i. 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 veranderte 

 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 sexueile Unterschiede zuriickzufuhren. 



"5. Die 'anormalen' Wohnkammern und die mit denselben 



