Species inquirendae 66 



Clinostomum (?) incommodum Leidy 60 



Monostomum ornatum Leidy 66 



Monostomum spatulatum Leidy 68 



Monostomum affine Leidy 68 



Monostomum asperum Vaillant 69 



Monostomum amiuri Stafford 70 



The polyphyletic origin of the monostomes 72 



Introduction 72 



Interrelationship of the monostome families 74 



Apparent relation of these families to other groups 75 



Summary 80 



Bibliography 82 



Description of plates 88 



