5^7, 3 



UNITED STATES GEOLOGICAL SURVEY 



GEORGE OTIS SMITH, Director 



THE 



MESOZOIC AID CMOZOIC ECHINODERMATA 

 OF THE UNITED STATES 



BY 



WILLIAM BULLOCK CLARK 



AND 



MAYVILLE W. TWITCHELL 



WASHINGTON 



GOVERNMENT PRINTING OFFICE 



1915 



