vi. PREFACE. 



heretofore not known in rocks of a more ancient date than those 

 of the higher portions of Upper Silurian, made its appearance or 

 existed in the Lower Silurian period. 



In order to render the work more useful to the student I have 

 given an introductory section, containing a short account of the 

 history and structure of the Crinoideae. 



E. BILLINGS. 



