126 Diener — Critical Phase in History of Ammonites. 



Lack of Internal Adaptation and Inadaptability. — 

 Arcestes is one of the most persistent ammonite genera, 

 ranging from the Anisic into the Khsetic stage, without 

 undergoing any modifications of its characters. Thus its 

 inadaptability can scarcely have been the primary cause 

 of its extinction. 



In the family of Ceratitidae Hyatt signalized the first 

 symptoms of general regression by the appearance of 

 uncoiled and turriliticonic genera in the Noric stage. 

 The development of uncoiled shells, which reaches its 

 climax in the lower and middle Cretaceous, is considered 

 as a sign of degeneration by many palaeontologists. To 

 this view I cannot assent. The best instance of this mode 

 of development is Lytoceras. It is one of the most con- 

 servative types from the Lias up to the Neocomian, when 

 suddenly a large number of uncoiled, straight, hook- 

 shaped and even turriliticonic genera branch off from the 

 old stem. All this stock flourishes for a considerable 

 time. This does not mean degeneration, but adaptation 

 to new and different forms of marine life, from a ben- 

 thonic swimming to a chiefly creeping or even sessile 

 mode of living. Still less can Lytoceras be stigmatized 

 as degenerating, if we take into consideration the fact 

 that it survives all its uncoiled offspring and persists 

 into the Senonian stage. I consequently find in the devel- 

 opment of uncoiled shapes in the family Ceratitidae, which 

 begins with Choristoceras in the upper Carnic and reaches 

 its climax in the lower Noric substage, a sign of increased 

 adaptability to new modes of life, not of degeneration. 



Peculiarities of Constitution. — It is only touching the 

 fringe of a great subject, if I venture to call attention to 

 Brocchi's hypothesis, that the gradual and successive 

 disappearance of species might be regulated by a constant 

 law, that their death, like that of individuals, might 

 depend on certain peculiarities of constitution. In our 

 special case, as in many others, the extermination of a 

 large and flourishing group of animals can be explained 

 satisfactorily neither by external nor by such internal 

 causes as are accessible to examination. In such cases 

 Brocchi's hypothesis, although dealing with powers and 

 influences of a still hitherto obscure nature, may yet serve 

 its purpose as a first attempt to approach the solution of 

 a hitherto unexplained problem. 



University of Vienna. 



