676 



MOLLUSCA 



PHYLÜM VI 



Piilclielliidae and Eiigonoceratidae, which were probably connected witli tlie younger 

 Cosnioceratidac (Hoplites). The Cretaceous Ammonitic fauna derives a special 

 character from tlie great development of aberrant forms, whicli are most abundant in 

 the Neocomian, but in part last into the higher stages of the Cretaceous. The genera 

 Macroscaphües, Pictetia, Hamües, Anisoceras, Turrilites, BacuUtes, Grioceras and 

 Scaphites are confined exclusively to the Cretaceous. 



The sudden extinction of the Ammonites at the end of the Cretaceous is one of 



Diagram showing the Relationships of Paleozoic and 

 Early Mesozoic Ammonoids 



Tropitoidea 



Ptychiloidea Lytoceratoidea 



Pinacoceratoidea 



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the most reniarkable phenomena in the history of the organic world, and one as yet 

 witliout explanation. Great changes in conditions of life must have taken place at 

 the border between Cretaceous and Tertiary to have brought about the extinction of 

 this flourishing and highly organised group of animals not only in Europe, but also 

 in all other parts of the world. 



The System of nomenclature now commonly in vogue has placed obstacles in the 

 way of a clear view of the general characters of the whole group of Ammonoids. 

 And this is especially unfortunate because in more recent years the genera and 

 families named have been difficult to distinguish, and in many cases have received 

 very vague definitions. At present the prevailing tendency is to subdivide rather 

 than to unite, and some authors are in a fair way to change every time-honoured 

 species into a special genus or family. 



Few divisions of the animal kingdom have left such a perfect record of their 



