231] NORTH AMERICAN MONOSTOMES 13 



FAMILIES OF MONOSTOMIDAE 



Altho in another section of this paper the monostomes are shown to 

 be more closely related to other groups than to each other it is thought 

 best to preserve the group classification until further evidence is secured 

 which definitely proves the suggested relationship. For this reason the 

 key to the monostome families is included and it is expected to serve only 

 for the rapid determination of specimens rather than to furnish full diag- 

 nostic characters. 



KEY TO FAMILIES 



1 (2) Intestinal crura anastomosing in the posterior end 



Family Cyclocoelidae 



2(1) Intestinal crura ending blindly near the posterior end 3 



3(4) Testes lateral to crura Family Notocotylidae 



4(3) Testes within crura 5 



5(6) Excretory pore posterior and terminal .... Family Collyriclidae 

 6(5) Excretory pore dorsal; in anterior body half . Family Heronimidae 



