^oy 19 nm 



THE DEVELOPMENT OF LYTOCERAS AND 



PHYLLOCERAS. 



BY JAMES PERRIN SMITH. 



CONTENTS. 



Page. 



Plates XVI-XX. 



Introduction 129 



Nomenclature of Stages of Growth 131 



Law of Acceleration of Development 133 



Lytoceratid^ 135 



Lytoceras 136 



Lytoceras alamedense, sp. nov 136 



Ontogeny 138 



Phylembryonic Stage 138 



Larval Stages 140 



Adolescent Stages 143 



Ephebic Stage 144 



Table of Stages of Growth 145 



Phylloceras 145 



PhYLLOCERAS ONOiENSE St ANTON I47 



Ontogeny 147 



Table of Stages of Growth 149 



Conclusion 149 



Explanation of the Figures 152 



Introduction. 



Lytoceras and Phylloceras are exceedingly interesting 

 because they are simple, unspecialized genera, long-lived, 

 little changed, and yet with the power of giving off other 

 variable branches. They are the two longest lived genera 

 of ammonites, ranging from about the end of the Trias to 

 the Upper Cretaceous, at least seven millions of years by a 

 conservative estimate. According to most authorities these 

 are the only groups of ammonites in the Jura that do not 

 belong to the stock of the Arietidce, and according to 

 others they are the ancestors of even that stock through 

 Psiloceras. 



It is in these old, unspecialized genera that we must seek 

 to unravel the genealogical tangle, either in the older for- 



[ 129 ] August 29, 1898. 



