64 ILLINOIS BIOLOGICAL MONOGRAPHS [64 



maycush and consequently some of the best diagnostic features could 

 not be determined it seems best to consider these forms identical. 



Proteocephalus pusUlus Ward is closely allied to P. exiguus La Rue 

 but is differentiated from that species by its larger head, larger suckers, 

 and larger sucker openings. The measurements of proglottids of the 

 two species are much alike. However, the type of segmentation is very 

 dissimilar. In the reproductive organs P. pusillus varies from P. exi- 

 guus in having more numerous and larger testes. The testes of P^ 

 pjisillus lie in two layers while in P. exiguus they are in one layer. The 

 cirrus-pouch of P. exiguus is much longer than that of P. pusillus. The 

 ovaries of the two species are very different. In the one species the 

 lobes are slightly arched while in the other the lobes are bent nearly 

 together. The lateral uterine pouches of P. exiguus are larger and 

 scarcely as numerous as in P. pusillus. P. pusillus somewhat resembles 

 P. fallax La Rue but it is smaller. It has a larger head and suckers, 

 more numerous and larger testes, a much shorter cirrus-pouch, and 

 more numerous uterine outpocketings than P. fallax. The ovaries of 

 the two species are very different. 



P. pusillus differs from P. agonis Barbieri in having a fifth sucker. 

 Moreover P. pusillus is a larger form than P. agonis. P. agonis has 

 smaller and more numerous testes than P. pusillus, and also fewer 

 uterine outpocketings. P. pusillus differs radically in size and propor- 

 tions from P. percae (0. F. Miiller). P. pusillus differs from P. esocis 

 (Schneider) in having a larger head, larger suckers, in possessing a 

 fifth sucker, and a shorter neck. The proportions of the segments are 

 different. The length of the cirrus-pouch and the ratio of its length to 

 the proglottid breadth differ greatly in the two species. The character 

 of the segmentation is very unlike. There is very little probability that 

 P. pusillus is the same as P. salvelini (Linton). P. salvelini has no fifth 

 sucker. Its head, suckers, and cirrus-pouch are larger than those of 

 P. pusillus. Linton's description of P. salvelini does not furnish many 

 characters for determination and his drawing of the proglottid is so 

 indistinct that it cannot be relied upon as a source of data. 



