TABLE OF CONTENTS 



PAGE 



Introduction 7 



Polystomidae 9 



Historical Review of the Family g 



The Genus Polystoma i6 



Anatomy and Histology of the Polystomidae 19 



Polystoma orbiculare Stunkard 1916 31 



Polystoma opacum Stunkard 1916 _ 34 



Polystoma megacotyle Stunkard 1916 37 



Polystoma microcotyle Stunkard 1916 39 



Polystoma coronatum Leidy 1888 40 



Polystoma liassalli Goto 1899 42 



Polystoma oblongum Wright 1879 44 



Key to the Species of the Genus Polystoma 46 



Aspidogastridae 47 



Aspidogaster conchicola von Baer 1827 48 



Cotylaspis insignis Leidy 1856 49 



Cotylaspis cokeri Barker and Parsons 1914. SO 



Classification of the Family 57 



Paramphistomidae _ 60 



Historical Review of the Family 60 



The Genus Alassostoma 64 



Alassostoma magnum Stunkard 1916. 66 



Alassostoma parvum Stunkard 1916 69 



The Genus Zygocotyle _ 71 



Zygocotyle ceratosa Stunkard I9i6._ 72 



Classification of the Family 75 



Relation of the Families to the Order 80 



List of New Species 84 



Bibliography 85 



Explanation of Plates _ 91 



